mtd: rawnand: Annotate implicit fall through in nand_command/nand_command_lp

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1).

This commit removes the following warnings:

  drivers/mtd/nand/raw/nand_legacy.c:332:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
  drivers/mtd/nand/raw/nand_legacy.c:483:3: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>