iw_cxgb3: initialize ibdev.iwcm->ifname for port mapping

The IWCM uses ibdev.iwcm->ifname for registration with the iwarp
port map daemon.  But iw_cxgb3 did not initialize this field which
causes intermittent registration failures based on the contents of the
uninitialized memory.

Fixes: c1340e8a ("iw_cxgb3: support for iWARP port mapping")
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>