staging: comedi: ii_pci20kc: cleanup the ai subdevice

The PCI-200341M-1 module can both provide an analog input subdevice
in this driver. The module provides four differential input channels
with four programmable gains.

Currently the gain is configured as an option passed when the board
is attached. Rewrite the ai subdevice support functions to properly
handle the programmable gain.

For aesthetics, redefine the memory map for the module.
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>