switch: statement may fall through

[kuldeepdhaka]

gcc warning:

../../usbd/usbd_ep0.c: In function 'standard_get_descriptor':
../../usbd/usbd_ep0.c:167:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (index < dev->info->device.desc->bNumConfigurations) {
      ^
../../usbd/usbd_ep0.c:173:2: note: here
  case USB_DT_STRING:
  ^~~~

https://github.com/insane-adding-machines/unicore-mx/blob/master/lib/usbd/usbd_ep0.c#L394