kaweth: Clean up code

The driver kaweth yields a -EBUSY error when starting, and a -ETIME
error when shutting down. These errors are avoided, and the RX status
is further checked for other potential errors.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>