RDMA/iw_cxgb4: fix the calculation of ipv6 header size

Take care of ipv6 checks while computing header length for deducing mtu
size of ipv6 servers. Due to the incorrect header length computation for
ipv6 servers, wrong mss is reported to the peer (client).

Signed-off-by: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>