spl: mmc: properly annotate fallthrough

spl: mmc: properly annotate fallthrough

Depending on the various MMC boot configurations, we might end up with
trying filesystem mode when a raw image boot failed. This fall-through
in the switch/case statement is explained in a comment, but this is not
visible to the compiler, which still will complain.

Add the proper compiler-visible annotation, to allow enabling the
compiler check in the future.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>