staging: comedi: range: introduce range_unipolar2_5

Introduce a simple unipolar 0 to 2.5 range, range_unipolar2_5, for
use by the comedi drivers.

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>