mtd: rawnand: jz4780: annotate implicit fall throughs

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings. Fix them up.

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