scsi_dh_alua: sanitze sense code handling

The only check for a valid sense code is calling scsi_normalize_sense()
and check the return value. So drop the pointless checks and rely on
scsi_normalize_sense() to figure out if the sense code is valid.
With that we can also remove the 'senselen' field.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.com>
Reviewed-by: Bart van Assche <bvanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>