usb: gadget: at91_udc: move at91_udc_data in at91_udc.h

struct at91_udc_data is now only used inside the driver, move it to its
include.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>