net/macb: better manage tx errors

Handle all TX errors, not only underruns. TX error management is
deferred to a dedicated workqueue.
Reinitialize the TX ring after treating all remaining frames, and
restart the controller when everything has been cleaned up properly.
Napi is not stopped during this task as the driver only handles
napi for RX for now.
With this sequence, we do not need a special check during the xmit
method as the packets will be caught by TX disable during workqueue
execution.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>