IB/mlx4: Fix handling of wq->tail for send completions

Cast the increment added to wq->tail when send completions are
processed to u16 to avoid using wrong values caused by standard
integer promotions.

The same bug was fixed in libmlx4 by Eli Cohen <eli@mellanox.co.il>.

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