at91_udc.c: Fix unused variable warning

With gcc-6 and later we see warnings that at91sam9263_udc_caps and
at91rm9200_udc_caps are unused.

Fixes: 62019767 ("usb: gadget: at91_udc: add at91_udc into U-Boot")
Cc: Lukasz Majewski <lukma@denx.de>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Heiko Schocher <hs@denx.de>