scsi: qla4xxx: ql4_os: mark expected switch fall-through

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

Notice that, in this particular case, I replaced "allow fall-through" with
a "fall through" annotation, which is what GCC is expecting to find.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Nilesh Javali <njavali@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>