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

This patch fixes the following warnings:

drivers/net/wireless/realtek/rtw88/phy.c: In function ‘rtw_get_channel_group’:
./include/linux/compiler.h:77:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/asm-generic/bug.h:125:2: note: in expansion of macro ‘unlikely’
  unlikely(__ret_warn_on);     \
  ^~~~~~~~
drivers/net/wireless/realtek/rtw88/phy.c:907:3: note: in expansion of macro ‘WARN_ON’
   WARN_ON(1);
   ^~~~~~~
drivers/net/wireless/realtek/rtw88/phy.c:908:2: note: here
  case 1:
  ^~~~
In file included from ./include/linux/bcd.h:5,
                 from drivers/net/wireless/realtek/rtw88/phy.c:5:
drivers/net/wireless/realtek/rtw88/phy.c: In function ‘phy_get_2g_tx_power_index’:
./include/linux/compiler.h:77:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/asm-generic/bug.h:125:2: note: in expansion of macro ‘unlikely’
  unlikely(__ret_warn_on);     \
  ^~~~~~~~
drivers/net/wireless/realtek/rtw88/phy.c:1021:3: note: in expansion of macro ‘WARN_ON’
   WARN_ON(1);
   ^~~~~~~
drivers/net/wireless/realtek/rtw88/phy.c:1022:2: note: here
  case RTW_CHANNEL_WIDTH_20:
  ^~~~

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

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>