stm32/usbdev: Fix USBD setup request handler to use correct recipient.

Prior to this patch the USBD driver did not handle the recipient correctly
for setup requests.  It was not interpreting the req->wIndex field in the
right way: in some cases this field indicates the endpoint number but the
code was assuming it always indicated the interface number.

This patch fixes this.  The only noticeable change is to the MSC
interface, which should now correctly respond to the USB_REQ_CLEAR_FEATURE
request and hence unmount properly from the host when requested.