amd-xgbe: Use system workqueue for device restart

A previous patch switched from using the system workqueue to the device
workqueue for various operations. During a device restart the device
workqueue is flushed so the restart cannot use this workqueue or else
a deadlock results.  Move the device restart back to using the system
workqueue.

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