staging: comedi: adl_pci9118: refactor 'rangelist_ai' in boardinfo

The boards supported by this driver either have "normal" or "high gain"
analog input ranges.

For aesthetics, replace the 'rangelist_ai' member in the boardinfo with
a bit-field flag 'is_hg' to indicate the "high gain" range is used.
Refactor the subdevice init to use the flag to set the correct range_table.

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>