Bluetooth: Mesh: Call bt_mesh_send_cb.end cb by the end of adv

Before this change, the bt_mesh_send_cb.end callback was called when all
references to the adv were removed. If an implementation kept more
references to the adv buffer after calling `bt_mesh_adv_send`, the end
callback would not be called when the advertiser finished advertising
this adv.

With this change, the end callback is always called by the advertiser
when the advertisement is finished regardless of the number of
references. This allows an implementation to keep the adv buffer for the
future use. As an example, pb_adv.c keeps advs for retransmission.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>