IB/mlx4: Increase max outstanding RDMA reads as target

Change the maximum number of outstanding RDMA reads allowed as a
target from 4 to 16 to per QP.  This allows RDMA read operations to
pipeline better.

Pointed out by Dotan Barak and Sagi Rotem.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>