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

This patch fixes the following warnings:

drivers/mtd/chips/cfi_cmdset_0002.c: In function ‘get_chip’:
drivers/mtd/chips/cfi_cmdset_0002.c:870:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (mode == FL_READY && chip->oldstate == FL_READY)
      ^
drivers/mtd/chips/cfi_cmdset_0002.c:873:2: note: here
  default:
  ^~~~~~~
drivers/mtd/chips/cfi_cmdset_0002.c: In function ‘cfi_amdstd_sync’:
drivers/mtd/chips/cfi_cmdset_0002.c:2745:16: warning: this statement may fall through [-Wimplicit-fallthrough=]
    chip->state = FL_SYNCING;
    ~~~~~~~~~~~~^~~~~~~~~~~~
drivers/mtd/chips/cfi_cmdset_0002.c:2750:3: note: here
   case FL_SYNCING:
   ^~~~

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

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Tokunori Ikegami <ikegami.t@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>