gianfar: Wait for both RX and TX to stop

When gracefully stopping the controller, the driver was continuing if
*either* RX or TX had stopped.  We need to wait for both, or the
controller could get into an invalid state.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>