rp2: Don't disable USB if going to DORMANT mode.

In this mode, XOSC is stopped so can't really keep
the USB PLL enabled.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

ports/rp2/modmachine.c

@@ -138,7 +138,8 @@ static void mp_machine_lightsleep(size_t n_args, const mp_obj_t *args) {
 
     #if MICROPY_HW_ENABLE_USBDEV
     // Only disable the USB clock if a USB host has not configured the device
-    bool disable_usb = !tud_mounted();
+    // or if going to DORMANT mode.
+    bool disable_usb = !(tud_mounted() && n_args > 0);
     #else
     bool disable_usb = true;
     #endif