staging: comedi: adq12b: fix ctreg (ai channel/range) programming

This driver only updates the ctreg (ai channel/range) register if the
desired channel or range has changed since the last analog input read
operation. It does this becuase the hardware requires an udelay to
allow the multiplexor to settle. Unfortunatly the current code never
updates the 'last_channel' and 'last_range' in the private data so
the ctreg gets updated every time.

Fix this and simplify it a bit by just storing the last ctreg value
in the private data.

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>