Staging: comedi: pcl812: Update MUX for multi-channel AI command

For multi-channel AI commands, the interrupt handler is missing code to
switch to the next channel.  Add some (untested) code to handle this.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>