usb: gadget: Move gadget functions code

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

Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: I1eab0190f8d42e3be1b4e91ad3bc3a2dc853b0ef