target/cxgbit: use cxgb4_tp_smt_idx() to get smt idx

cxgb4_tp_smt_idx() returns smt idx for T4,T5,T6 adapters.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>