Fix the rx code path that does not handle the full rx ring correctly.

When the rx ring is set to the max. size (i.e. 255), the consumer and
producer indices will be the same when completing an rx packet. Fix
the rx code to handle this condition properly.
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>