mtd: cfi_util: mark expected switch fall-throughs

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_util.c: In function ‘cfi_build_cmd’:
drivers/mtd/chips/cfi_util.c:110:10: warning: this statement may fall through [-Wimplicit-fallthrough=]
   onecmd |= (onecmd << (chip_mode * 32));
   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/chips/cfi_util.c:112:2: note: here
  case 4:
  ^~~~
drivers/mtd/chips/cfi_util.c:113:10: warning: this statement may fall through [-Wimplicit-fallthrough=]
   onecmd |= (onecmd << (chip_mode * 16));
   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/chips/cfi_util.c:114:2: note: here
  case 2:
  ^~~~
drivers/mtd/chips/cfi_util.c: In function ‘cfi_merge_status’:
drivers/mtd/chips/cfi_util.c:163:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   res |= (onestat >> (chip_mode * 32));
   ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/chips/cfi_util.c:165:2: note: here
  case 4:
  ^~~~
drivers/mtd/chips/cfi_util.c:166:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   res |= (onestat >> (chip_mode * 16));
   ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mtd/chips/cfi_util.c:167:2: note: here
  case 2:
  ^~~~

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: Kees Cook <keescook@chromium.org>
Signed-off-by: Richard Weinberger <richard@nod.at>