USB: <linux/usb/ch9.h> minor doc update

Minor doc update to <linux/usb/ch9.h> ... say where USB_DT_CS_* came
from and update the definitions to match how they're derived there.

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