[appsync] Fix condition for logging systemd service stop problems

The systemd_control_service() function returns zero on success. Due to
incorrect boolean check in appsync_stop() diagnostic failure messages are
emitted on success and failures are ignored silently.

Fix the condition so that failures are logged.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>

src/usb_moded-appsync.c

@@ -456,7 +456,7 @@ int appsync_stop(void)
 
     if(data->systemd)
     {
-        if(!systemd_control_service(data->name, SYSTEMD_STOP))
+        if(systemd_control_service(data->name, SYSTEMD_STOP))
 		log_debug("Failed to stop %s\n", data->name);
     }
   }