[PATCH] USB: Kconfig fixes for usb/gadget

This prevents gadget drivers from being selected when no controller has
been selected, by adding an additional boolean and depending on it.
It's mostly to help "allmodconfig".

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