Instead of iterate to find usb_udc according to usb_gadget, this
way is easier.

Alan Stern suggests this way too:
http://marc.info/?l=linux-usb&m=142168496528894&w=2Acked-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NPeter Chen <peter.chen@freescale.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>