ANDROID: usb: gadget: configfs: Add usb_function ptr to fi struct

Add a pointer to the usb_function inside the
usb_function_instance structure to service
functions specific setup requests even before
the function gets added to the usb_gadget

Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: I6f457006f6c5516cc6986ec2acdf5b1ecf259d0c