staging: comedi: das1800: refactor 'range_ai' boardinfo

The boards supported by this driver have analog inputs with gains
of 1, 10, 50, and 250 ("01" type) or 1, 2, 4, and 8 ("02" type).
Unfortunately, the "01"/"02" type cannot be determined from the
boards id or by probing.

Replace the 'range_ai' member of the boardinfo with a bit-field flag,
'is_01_series' and only set it for the "01" type boards. Remove the
unnecessary initialization for the "02" type boards.

For aesthetics, rename the comedi_lrange arrays and document the gain.

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