net: yaip: Make sure that RX is started before TX

The network stack initialization will need to make sure
that RX side is ready before TX side is started. So in
order to do that the TX init is called from RX fiber.

This is needed so that we can start to send network packets already
during the net_init(), like IPv6 neighbor solicitation messages
during duplicate address detection process.

Change-Id: I12fde3d388d9aab41daa93b1edc02f25b8c5511d
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>