xen: Drop net_rx_ok

Let net_rx_packet() (which checks the same conditions) drops the packet
if the device is not ready. Drop net_xen_info.can_receive and update the
return value for the buffer full case.

We rely on the qemu_flush_queued_packets() in net_event() to wake up
the peer when the buffer becomes available again.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1438077176-378-1-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>