iw_cxgb4: initialize ibdev.iwcm->ifname for port mapping

The IWCM uses ibdev.iwcm->ifname for registration with the iwarp
port map daemon.  But iw_cxgb4 did not initialize this field which
causes intermittent registration failures based on the contents of the
uninitialized memory.

Fixes: 170003c8 ("iw_cxgb4: remove port mapper related code")
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>