net: Use net_buf APIs for getting net_buf elements from FIFO

The net_buf API ensures that the frags pointer gets initialized
properly. Using nano_fifo_get could mean that buf->frags is something
else than NULL if there was more than one item in the FIFO.

Change-Id: If65e85a7cbe82f562307dc781d48110c3be0472b
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>