[USB] Enable VBUS sensing for OTG_FS

Previously, this was enabled only for OTG_HS, for some reason. This
probably needs to be more configurable by board, but this is enough to
for the specific board we are working with now.

See stm32duino#886

Author: matthijskooijman

cores/arduino/stm32/usb/usbd_conf.c

@@ -486,7 +486,7 @@ USBD_StatusTypeDef USBD_LL_Init(USBD_HandleTypeDef *pdev)
   g_hpcd.Instance = USB_OTG_FS;
   g_hpcd.Init.use_dedicated_ep1 = DISABLE;
   g_hpcd.Init.dma_enable = DISABLE;
-  g_hpcd.Init.vbus_sensing_enable = DISABLE;
+  g_hpcd.Init.vbus_sensing_enable = ENABLE;
   g_hpcd.Init.use_external_vbus = DISABLE;
 #else
   g_hpcd.Instance = USB;