scsi: qla2xxx: Suppress gcc 7 fall-through warnings

Avoid that building with gcc 7 and W=1 triggers warnings similar to the
following:

drivers/scsi/qla2xxx/qla_isr.c:1189:27: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Himanshu Madhani <himanshu.madhani@cavium.com>
Cc: Quinn Tran <quinn.tran@cavium.com>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>