IB/mlx4: Make sure RQ allocation is always valid

QPs attached to an SRQ must never have their own RQ, and QPs not
attached to SRQs must have an RQ with at least 1 entry.  Enforce all
of this in set_rq_size().

Based on a patch by Eli Cohen <eli@mellanox.co.il>.

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