bnxt_re: Fix update of qplib_qp.mtu when modified

The MTU value in the qplib_qp.mtu should be
consistent with whatever mtu was set during
INIT to RTR.The Next PSN and number of packets
are calculated based on this member in the qplib_qp structure.

Signed-off-by: Narender Reddy <narender.reddy@broadcom.com>
Signed-off-by: Devesh Sharma <devesh.sharma@broadcom.com>
Signed-off-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>