staging: comedi: das800: use comedi provided range_bipolar5

Remove the private range, range_das800_ai, in this driver and use
the comedi provided range_bipolar5 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>