Bluetooth: Mesh: Fix calling complete callback for bt_mesh_provision()

If the app does direct provisioning, it may still want to do common
handling through its provisioning complete callback (if it has one
registered). This also means that we always require a non-NULL
provisioning context provided to bt_enable(), and that it needs to
fail if NULL was given.

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