RDMA/cxgb4: Fix ntuple calculation for ipv6 and remove duplicate line

This fixes ntuple calculation for IPv6 active open request for T5
adapter.  And also removes an duplicate line which got added in commit
92e7ae71 ("iw_cxgb4: Choose appropriate hw mtu index and ISS for
iWARP connections")

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>