dpaa2-eth: destroy workqueue at the end of remove function

The commit c5521189 ("dpaa2-eth: support PTP Sync packet one-step
timestamping") forgets to destroy workqueue at the end of remove
function.

Fix this by adding destroy_workqueue before fsl_mc_portal_free and
free_netdev.

Fixes: c5521189

 ("dpaa2-eth: support PTP Sync packet one-step timestamping")
Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>