staging: comedi: rti800: remove adc_range from private data

The 'adc_range' is only used in the attach of the board. Remove it
from the private data.

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>