Bluetooth: Mesh: Call bt_mesh_reset in cfg srv from workqueue

Struct bt_mesh_send_cb.end is called from the mesh advertiser thread.
bt_mesh_reset now requires more stack size, thus can't be called from
the advertiser thread. k_work instance requires less memory than needed
for bt_mesh_reset.

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