usb: gadget: Relocate f_accessory

3.18 kernel has reorganized drivers/usb/gadget
directory. Moving accessory gadget driver from
drivers/usb/gadget to drivers/usb/gadget/function

Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: If73c6df0537c4b1f51338ed3b0db817e51f06b4a