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

Bug: 63740241
Bug: 68755607
Bug: 78114713
Bug: 120441124
Change-Id: I6f457006f6c5516cc6986ec2acdf5b1ecf259d0c
[badhri: This is a supporting patch for other patches which have
         replacements pipelined. It can be dropped when those
         implementations land.]
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>