scsi: ppa: 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
Addresses-Coverity-ID: 114989
Addresses-Coverity-ID: 114990
Addresses-Coverity-ID: 114991
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>