[IB] mthca: don't disable RDMA writes if no responder resources

Responder resources are only required to handle RDMA reads and atomic
operations, not RDMA writes.  So the driver should allow RDMA writes
even if responder resources are set to 0.  This is especially
important for the UC transport -- with the old code, it was impossible
to enable RDMA writes for UC QPs.

Signed-off-by: Roland Dreier <rolandd@cisco.com>