net: yaip: Add nbuf buffer API

User should use the net_nbuf API that is supporting a concept
of RX, TX and DATA buffers, instead of using directly the net_buf
low level API.

Change-Id: I3b8f5f13cd37d42d2322a58b35573e117d520c54
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>