usb: gadget: protect gadget_chips.h from been included twice

Without it, we might have trouble when trying to write
some composite gadget drivers.

Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>