usb: make "nousb" a clear module parameter

It shouldn't matter how usbcore is compiled. As it is a subsystem,
the correct way to use nousb should be usbcore.nousb

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>