In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/mtd/nand/raw/diskonchip.c: In function ‘doc_probe’:
./include/linux/printk.h:303:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/nand/raw/diskonchip.c:1479:4: note: in expansion of macro ‘pr_err’
    pr_err("DiskOnChip Millennium Plus 32MB is not supported, ignoring.\n");
    ^~~~~~
drivers/mtd/nand/raw/diskonchip.c:1480:3: note: here
   default:
   ^~~~~~~
drivers/mtd/nand/raw/nandsim.c: In function ‘ns_init_module’:
drivers/mtd/nand/raw/nandsim.c:2254:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
    chip->bbt_options |= NAND_BBT_NO_OOB;
drivers/mtd/nand/raw/nandsim.c:2255:2: note: here
  case 1:
  ^~~~
drivers/mtd/nand/raw/nuc900_nand.c: In function ‘nuc900_nand_command_lp’:
./arch/x86/include/asm/io.h:91:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 #define __raw_writel __writel
drivers/mtd/nand/raw/nuc900_nand.c:52:2: note: in expansion of macro ‘__raw_writel’
  __raw_writel((val), (dev)->reg + REG_SMCMD)
  ^~~~~~~~~~~~
drivers/mtd/nand/raw/nuc900_nand.c:196:3: note: in expansion of macro ‘write_cmd_reg’
   write_cmd_reg(nand, NAND_CMD_READSTART);
   ^~~~~~~~~~~~~
drivers/mtd/nand/raw/nuc900_nand.c:197:2: note: here
  default:
  ^~~~~~~
drivers/mtd/nand/raw/omap_elm.c: In function ‘elm_context_restore’:
drivers/mtd/nand/raw/omap_elm.c:512:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
    elm_write_reg(info, ELM_SYNDROME_FRAGMENT_4 + offset,
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      regs->elm_syndrome_fragment_4[i]);
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/nand/raw/omap_elm.c:514:3: note: here
   case BCH8_ECC:
   ^~~~
drivers/mtd/nand/raw/omap_elm.c:517:4: warning: this statement may fall through [-Wimplicit-fallthrough=]
    elm_write_reg(info, ELM_SYNDROME_FRAGMENT_2 + offset,
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      regs->elm_syndrome_fragment_2[i]);
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/nand/raw/omap_elm.c:519:3: note: here
   case BCH4_ECC:
   ^~~~
drivers/mtd/nand/raw/omap_elm.c: In function ‘elm_context_save’:
drivers/mtd/nand/raw/omap_elm.c:466:37: warning: this statement may fall through [-Wimplicit-fallthrough=]
    regs->elm_syndrome_fragment_4[i] = elm_read_reg(info,
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
      ELM_SYNDROME_FRAGMENT_4 + offset);
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/nand/raw/omap_elm.c:468:3: note: here
   case BCH8_ECC:
   ^~~~
drivers/mtd/nand/raw/omap_elm.c:471:37: warning: this statement may fall through [-Wimplicit-fallthrough=]
    regs->elm_syndrome_fragment_2[i] = elm_read_reg(info,
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
      ELM_SYNDROME_FRAGMENT_2 + offset);
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/nand/raw/omap_elm.c:473:3: note: here
   case BCH4_ECC:
   ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>