scsi: scsi_dh_alua: Correct comment for alua_alloc_pg()

In the comment for function alua_alloc_pg() the argument '@h' is
mistakenly referred to.  Fix this by replacing it with the correct
argument reference, '@tpgs', and provide a short description.

Signed-off-by: John Pittman <jpittman@redhat.com>
Reviewed-by Laurence Oberman <loberman@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>