staging: comedi: range: introduce some simple mA ranges

The simple mA ranges 0 to 20, 4 to 20, and 0 to 32 are fairly common.
Introduce them in the comedi core and use them in the 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>