USB: gadget: atmel_usba: allow multi instance

drop static struct usba_udc the_udc

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>