IB/mlx5: Fix possible array overflow

The check to verify that userspace does not provide an invalid index to the
micro UAR was placed too late. Fix this by moving the check before using the
index.

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