usb: gadget: dummy_hcd: add support for USB_DT_BOS on rh

Without a reply for USB_DT_BOS the USB3 mode does not work since
448b6eb1 ("USB: Make sure to fetch the BOS desc for roothubs.).

Cc: stable@vger.kernel.org #v3.5
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Felipe Balbi <balbi@ti.com>