net: mvneta: Fix the case where the last poll did not process all rx

For the case where the last mvneta_poll did not process all
RX packets, we need to xor the pp->cause_rx_tx or port->cause_rx_tx
before claculating the rx_queue.

Fixes: 2dcf75e2 ("net: mvneta: Associate RX queues with each CPU")
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: David S. Miller <davem@davemloft.net>