ANDROID: usb: gadget: f_accessory: Move gadget functions code

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

[AmitP: Refactored the original changes after squashing of related patches]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>