bluetooth: hogp: Fix bt_hogp_rep_unsubscribe issue

The bt_hogp_rep_unsubscribe did not reset the rep->notify_cb,
while bt_hogp_rep_subscribe returned an error if
notify_cb was not null. Thus, it would be impossible
to subscribe again to the report notification after
unsubscribing.

This commit fixes this.

Signed-off-by: Artur Hadasz <artur.hadasz@nordicsemi.no>

Author: ahasztag

subsys/bluetooth/services/hogp.c

@@ -1013,6 +1013,7 @@ static uint8_t rep_notify_process(struct bt_conn *conn,
 			       struct bt_gatt_subscribe_params *params,
 			       const void *data, uint16_t length)
 {
+	int err = 0;
 	struct bt_hogp_rep_info *rep;
 
 	rep = CONTAINER_OF(params,
@@ -1033,7 +1034,13 @@ static uint8_t rep_notify_process(struct bt_conn *conn,
 		rep->size = (uint8_t)length;
 	}
 
-	return rep->notify_cb(rep->hogp, rep, 0, data);
+	err = rep->notify_cb(rep->hogp, rep, 0, data);
+
+	if (data == NULL) {
+		rep->notify_cb = NULL;
+	}
+
+	return err;
 }
 
 int bt_hogp_rep_subscribe(struct bt_hogp *hogp,