IB/mlx5: Fix send work queue size calculation

1. Make sure wqe_cnt does not exceed the limit published by firmware.

2. There is no requirement that the number of outstanding work
   requests will be a power of two. Remove the ilog2 in the
   calculation of sq.max_post to fix that.

3. Add case for IB_QPT_XRC_TGT in sq_overhead and return 0 as XRC
   target QPs do not have a send queue.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>