usbnet: Drop usbnet_can_receive

usbnet_receive already drops packet if rndis_state is not
RNDIS_DATA_INITIALIZED, and queues packet if in buffer is not available.
The only difference is s->dev.config but that is similar to rndis_state.

Drop usbnet_can_receive and move these checks to usbnet_receive, so that
we don't need to explicitly flush the queue when s->dev.config changes
value.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Message-id: 1436955553-22791-5-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
