usb: gadget: f_gsi: Fix NULL ptr dereference

When usb composition switch happens upon cable disconnect and
function bind fails, function's config pointer is set to NULL.
This is resulting in to NULL ptr dereference when config is
accessed from APIs which are called asynchronously and out
side of the usb core driver(i.e. dwc3) context. Fix the
issue by directly accessing gadget pointer from gsi driver
context in those APIs.

Change-Id: I1006881ae1838e8ddc8fa5e9ef501f4c658b54e7
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>