scsi: aic94xx: mark expected switch fall-throughs

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

Addresses-Coverity-ID: 114988 ("Missing break in switch")
Addresses-Coverity-ID: 114989 ("Missing break in switch")
Addresses-Coverity-ID: 114990 ("Missing break in switch")
Addresses-Coverity-ID: 114991 ("Missing break in switch")
Addresses-Coverity-ID: 114992 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>