usb: gadget: ffs: Multi-instance fix for use after free case

ffs can be used by many drivers, ADB can use it and MTP can
use it. Any other generic function can use it.

So we continue on the single instance patch, provide
multi-instance fix for the use-after-free issue.

Change-Id: I0056bd3779fb472b69e51391702b8b753d39372f
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>