cxgb4: Issue mbox commands on correct mbox

A couple of RDMA-related called to t4_query_params() were issuing mbox commands
on mbox0 instead of mbox4.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>