Fix sparse warnings generated from cfi_cmdset_0001.c.

drivers/mtd/chips/cfi_cmdset_0001.c:1783:5: warning: symbol 'cfi_intelext_erase_varsize' was not declared. Should it be static?
drivers/mtd/chips/cfi_cmdset_0001.c:2258:43: warning: Using plain integer as NULL pointer
Signed-off-by: NBen Dooks <ben-linux@fluff.org>
Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>