iw_cxgb4: Take clip reference before starting IPv6 listen

The h/w is designed in such a way that, if you do anything IPv6
related, a valid clip entry must be there. So take clip reference
before creating IPv6 listening servers, and then if we fail to
create server, release the clip entry.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>