axienet: Flush queued packets when rx is done

eth_can_rx checks s->rxsize and returns false if it is non-zero. Because
of the .can_receive semantics change, this will make the incoming queue
disabled by peer, until it is explicitly flushed. So we should flush it
when s->rxsize is becoming zero.

Squash eth_can_rx semantics into etx_rx and drop .can_receive()
callback, also add flush when rx buffer becomes available again after a
packet gets queued.

The other conditions, "!axienet_rx_resetting(s) &&
axienet_rx_enabled(s)" are OK because enet_write already calls
qemu_flush_queued_packets when the register bits are changed.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1436955553-22791-13-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>