scsi_dh_alua: do not fail for unknown VPD identification

Not every device will return a useable VPD identification, but still
might support ALUA. Rather than disable ALUA support we should be
allowing the device identification to be empty and attach individual
ALUA device handler to each devices.

[mkp: Fixed typo reported by Bart]

Reported-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Hannes Reinecke <hare@suse.com>
Tested-by: Paul Mackerras <paulus@ozlabs.org>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>