[SCSI] scsi_dh_alua: Re-enable STPG for unavailable ports

A quote from SPC-4: "While in the unavailable primary target port
asymmetric access state, the device server shall support those of
the following commands that it supports while in the active/optimized
state: [ ... ] d) SET TARGET PORT GROUPS; [ ... ]". Hence re-enable
sending STPG to a target port group that is in the unavailable state.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Babu Moger <babu.moger@netapp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>