USB: gadget: atmel_usba: move global struct usba_ep usba_ep to struct usba_udc

so we can have multiple usb gadget instance

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Bo Shen <voice.shen@atmel.com>