net: yaip: Do not remove fragments if main buffer is not removed

The fix prevents the net_nbuf_unref() from removing fragment
list if the top level net_buf is not removed.

Change-Id: Ice580c25bd74f3239c01321f2b9d61cc8c0c9c1d
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>