scsi_dh_alua: use scsi_vpd_tpg_id()

Use the common function 'scsi_vpd_tpg_id()' instead of open-coding
it in scsi_dh_alua.

[mkp: Applied by hand]

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