IB/mlx4: Fix a sparse endianness warning

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: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>