scsi: megaraid: megaraid_sas_base: Mark expected switch fall-through

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

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>