net: calxedaxgmac: fix race between xgmac_tx_complete and xgmac_tx_err

It is possible for the xgmac_tx_complete to run concurrently with
xgmac_tx_err since there are no locks. Fix this by moving the tx
error handling to a workqueue so we can disable napi while we reset
the transmitter.

Reported-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>