staging: comedi: made comedi_lrange struct constant

Added the const type qualifier to the comedi_lrange structure.

Signed-off-by: Artur Lorincz <larturus@yahoo.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>