USB: usb gadget stack can now -DDEBUG with Kconfig

Although the other USB driver directories got taught how use Kconfig
and the Makefile to enable the debugging messages enabled by -DDEBUG,
the gadget stack was overlooked.

This patch remedies that omission, but doesn't update any drivers to
remove previous idiosyncracies in this area ... other than the RNDIS
code, which defined its own DEBUG() macro in a broken way.

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