Bluetooth: Mesh: Add check to LPN friend_req_sent

Adds state check to the friend_req_sent CB to ensure
that the CB does not alter the LPN state if it is in a
disabled state. This resolves behavioural issue for lpn_set(false).

Signed-off-by: Anders Storrø <anders.storro@nordicsemi.no>
diff --git a/subsys/bluetooth/mesh/lpn.c b/subsys/bluetooth/mesh/lpn.c
index 684099e..35169a6 100644
--- a/subsys/bluetooth/mesh/lpn.c
+++ b/subsys/bluetooth/mesh/lpn.c
@@ -284,6 +284,10 @@
 		return;
 	}
 
+	if (lpn->state == BT_MESH_LPN_DISABLED) {
+		return;
+	}
+
 	lpn->adv_duration = duration;
 
 	if (IS_ENABLED(CONFIG_BT_MESH_LPN_ESTABLISHMENT)) {