usb: gadget: at91_udc: fix endpoint descriptor dereference

The patch 5a6506f0 (Update at91_udc to use usb_endpoint_descriptor inside the
struct usb_ep) removes the desc field of struct at91_ep. This convertion had
not been completed which leads to a compilation error.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>