pcnet: Drop pcnet_can_receive

pcnet: Drop pcnet_can_receive

pcnet_receive already checks the conditions and drop packets if false.
Due to the new semantics since 6e99c63 ("net/socket: Drop
net_socket_can_send"), having .can_receive returning 0 requires us to
explicitly flush the queued packets when the conditions are becoming
true, but queuing the packets when guest driver is not ready doesn't
make much sense.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Message-id: 1436955553-22791-3-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>