staging: comedi: rti800: use arrays to hold the ai/ao ranges

The analog in/analog out ranges are selected by the user using options
passed during the legacy attach. Put the valid ranges into arrays and
use those instead of the switch () statements when initializing the
subdevice range information.

If the passed user option is not valid, set the range information to
range_unknown.

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