IB/mlx4: Don't allocate RQ doorbell if using SRQ

If a QP is attached to a shared receive queue (SRQ), then it doesn't
have a receive queue (RQ).  So don't allocate an RQ doorbell (or map a
doorbell from userspace for userspace QPs) for that QP.

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