mtd: lpddr_cmds: Mark expected switch fall-through

mtd: lpddr_cmds: Mark expected switch fall-through

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

This patch fixes the following warnings:

drivers/mtd/lpddr/lpddr_cmds.c: In function ‘chip_ready’:
drivers/mtd/lpddr/lpddr_cmds.c:319:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (mode == FL_READY && chip->oldstate == FL_READY)
      ^
drivers/mtd/lpddr/lpddr_cmds.c:322:2: note: here
  default:
  ^~~~~~~

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>