usb: gadget: gsi: Avoid accessing an uninitialized variable

Prevent dereferencing an uninitialized f_gsi pointer.
Return a valid gsi context only after finding a valid
gsi_opts instance. Else return NULL.

Change-Id: I75578c0bc3f4673ec66cb80bcb7a6ae328489253
Signed-off-by: Krishna Kurapati <kriskura@codeaurora.org>