Staging: comedi: range: remove unnecessary sanity check

This check is unnecessary because range_table will always be
initialized to range_unknown by comedi_device_postconfig() for
drivers that do not initialize range_table or range_table_list

Signed-off-by: Fred Akers <knivey@botops.net>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>