broadband-modem-mbim: increase timeout for powering modem up and down

It's been observed that some modems occasionally take a long time to
power down (which may be due to some shutdown sequence that involves
communicating with network). This patch increases the timeout for
powering modem up and down from 10s to 20s.

src/mm-broadband-modem-mbim.c

@@ -1036,7 +1036,7 @@ common_power_up_down (MMIfaceModem *self,
     message = mbim_message_radio_state_set_new (state, NULL);
     mbim_device_command (device,
                          message,
-                         10,
+                         20,
                          NULL,
                          ready_cb,
                          result);