IB/mlx5: Flush cache workqueue before destroying it

Destroying the workqueue without flushing it first can lead to a case
in which the kernel tries to push a delayed work to the workqueue
which does not exist anymore.

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