amd-xgbe: Use device workqueue instead of system workqueue

The driver creates, flushes and destroys a device workqueue but queues
work to the system workqueue. Switch from using the system workqueue to
the device workqueue.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>