usb: gadget: at91_udc: fix build warning

commit e117e742 (usb: gadget: add "maxpacket_limit"
field to struct usb_ep) added a build warning to
at91_udc when it passed the wrong argument to
usb_ep_set_maxpacket_limit().

Fix this by passing correct argument.

Signed-off-by: Felipe Balbi <balbi@ti.com>