mtd: block2mtd: mark expected switch fall-throughs

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

Addresses-Coverity-ID: 402015 ("Missing break in switch")
Addresses-Coverity-ID: 402016 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>