Bluetooth: Mesh: Fix non-periodic Model Publication

The only generally available model supporting publication that's
convenient to be used for testing is the Health Server Model.
Unfortunately since this model supports period publication, the
non-periodic side got less attention and had some bugs.

The first thing that needs to be done is to verify that the period
returned by bt_mesh_model_pub_period_get() is positive. If it's zero
then no periodic publication should take place.

Another thing that this patch cleans up is the naming of the callback
used for periodic publishing. There's no need do require the callback
to call bt_mesh_model_publish() since this must happen no matter what,
so instead rename the callback from 'func' to 'update' and have the
access layer call bt_mesh_model_publish() if the callback was
successful.

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