usb gadget: link fixes for gadget zero

Change how the Gadget Zero 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>