staging: comedi: dt9812: use comedi provided range_unipolar2_5

Remove the private ranges, dt9812_2pt5_a{in,out}_range, in this
driver and use the comedi provided range_unipolar2_5 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>