staging: comedi: adv_pci1710: refactor ai range programming

The gain codes used to program the analog output range are currently
stored in const char arrays. The values look a bit "magic" and it's
not clear how they associate with the comedi_lrange without looking
through user manuals.

Refactor the ai range programming to clarify the driver and remove
the magic numbers. Also, refine the bits in the range register that
set the differential and unipolar modes.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>