usb: gadget: hid: don't STALL when processing a HID Descriptor request

This is a patch to fix an issue with the HID gadget which, at the moment,
returns STALL on a HID descriptor request. Essentially, the patch changes
the hid gadget such that a request for the HID descriptor is handled by
copying the descriptor into the response buffer, rather than falling
through the default case, in which the request is answered by a STALL.

Signed-off-by: Sebastian Bauer <mail@sebastianbauer.info>
Acked-by: Peter Korsgaard <peter.korsgaard@barco.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>