RDMA/cxgb4: Handle NET_XMIT return codes

cxgb4_create_server() and cxgb4_create_server6() return NET_XMIT_*
values or a negative errno. iw_cxgb4 need to handle this correctly.

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