tcm_qla2xxx: Convert to target_alloc_session usage

This patch converts existing qla2xxx target mode assignment
of struct qla_tgt_sess related sid + loop_id values to use
a callback via the new target_alloc_session API caller.

Cc: Himanshu Madhani <himanshu.madhani@qlogic.com>
Cc: Quinn Tran <quinn.tran@qlogic.com>
Cc: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>