staging: comedi: ii_pci20kc: cleanup the ao subdevice

The PCI-20006M-1 and PCI-20006M-2 modules can both provide an
analog output subdevice in this driver. Fix the module init code
to detect both of them.

The analog output can support +/-5, 0-10, or +/-10 volt ranges
depending on jumpers on the board. Report all the ranges to the
user instead of relying on the configuration options to set the
range. The user can then select the appropriate range depending
on how they have configured the hardware.

Tidy up the (*insn_{read,write})() functions to work like the
comedi core expects.

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>