macb: Handle Retry Limit Exceeded errors

When transfering large amounts of data we sometimes experienced that the
Retry Limit Exceeded (RLE) bit got set in TSR during transmission
attempts. When this happened the driver would stall in a state that
prevented any more data from being sent.

Signed-off-by: Erik Waling <erik.waling@konftel.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>