staging: comedi: das1800: use comedi provided range_bipolar10

Remove the private range, range_ao_1, in this driver and use the
comedi provided range_bipolar10 instead.

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>