mtd: rawnand: Annotate implicit fall through in nand_scan_tail

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_base.c:5556:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
  drivers/mtd/nand/raw/nand_base.c:5575:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
  drivers/mtd/nand/raw/nand_base.c:5613:13: 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>