IB/mlx4: Fix queue overflow check in post_recv

In mlx4_ib_post_recv(), we should check the queue for overflow using
recv_cq instead of send_cq (current code looks like a copy-and-paste
mistake).

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>