usb gadget: link fixes for storage gadget

Change how the file storage gadget driver builds:  don't
use separate compilation, since it works poorly when key
parts are library code (with init sections etc).  Instead
be as close as we can to "gcc --combine ...".

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>