Fix the following warning for the mlx4 driver:

    $ make M=drivers/infiniband C=2 CF=-D__CHECK_ENDIAN__
    drivers/infiniband/hw/mlx4/qp.c:1885:31: warning: restricted __be16 degrades to integer
Signed-off-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NRoland Dreier <roland@purestorage.com>