USB: gadget: Make g_hid device class conform to spec.

HID devices should specify this in their interface descriptors, not in the
device descriptor. This fixes a "missing hardware id" bug under Windows 7 with
a VIA VL800 (3.0) controller.

Signed-off-by: Orjan Friberg <of@flatfrog.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>