Bluetooth: mesh: Fix status returned from mod_unbind

According to Mesh Specification v1.0 4.3.2.48 Config Model App Status:
"The Status Code shall be Success if the received request was redundant
(bind request of existing binding, or unbind of a non-existing binding),
with no further action taken."

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>