mtd: rawnand: nand_base: annotate switch/case fallthrough

mtd: rawnand: nand_base: annotate switch/case fallthrough

The raw NAND flash code uses an implicit switch/case fallthrough to
share code when dealing with different ECC modes, and also when handling
some read command.

Add our "fallthrough;" statement-like macro before the respective labels
in the NAND code, to avoid a warning when GCC's -Wimplicit-fallthrough
warning option is enabled.

This copies the fallthrough annotations that the original kernel code
gained, before this function got refactored there.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Michael Trimrachi <michael@amarulasolutions.com>