usb gadget: function activation/deactivation

Add a new mechanism to the composite gadget framework, letting
functions deactivate (and reactivate) themselves.  Think of it
as a refcounted wrapper for the software pullup control.

A key example of why to use this mechanism involves functions that
require a userspace daemon.  Those functions shuld use this new
mechanism to prevent the gadget from enumerating until those daemons
are activated.  Without this mechanism, hosts would see devices that
malfunction until the relevant daemons start.

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