usb: gadget: ffs: Use local copy of descriptors for userspace copy

USB cable can be disconnected (function disable) and function
descriptors can be freed while userspace daemon requesting for
descriptors copy to userspace. Avoid stale pointer copy by always
copying only local copy of desctiptors.

Change-Id: I16c01d22058e7148546f1ffbc5017520402eda97
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>