broadband-modem: fix type in the ready() for iface_modem_3gpp_sync()

When calling an async method ona given type, the convention is to use
the same type in the corresponding ready() method, so that we can use
it without additional casts on the finish().

src/mm-broadband-modem.c

@@ -11981,7 +11981,7 @@ iface_modem_time_sync_ready (MMIfaceModemTime *self,
 }
 
 static void
-iface_modem_3gpp_sync_ready (MMBroadbandModem *self,
+iface_modem_3gpp_sync_ready (MMIfaceModem3gpp *self,
                              GAsyncResult     *res,
                              GTask            *task)
 {
@@ -11990,7 +11990,7 @@ iface_modem_3gpp_sync_ready (MMBroadbandModem *self,
 
     ctx = g_task_get_task_data (task);
 
-    if (!mm_iface_modem_3gpp_sync_finish (MM_IFACE_MODEM_3GPP (self), res, &error))
+    if (!mm_iface_modem_3gpp_sync_finish (self, res, &error))
         mm_obj_warn (self, "3GPP interface synchronization failed: %s", error->message);
 
     /* Go on to next step */