staging: comedi: adv_pci1710: clarify the 'act_chanlist'

This driver saves the channel list of a scan in the private data and uses
that list to check analog input samples for data dropout.

Currently the channel numbers are shifted 12 bits so that they match the
channel number in the samples that are read from the board. All of the
shifts make the driver a bit harder to follow.

Store the channel numbers directly to the 'act_chanlist' and shift the
sample value instead when checking for the dropout. Add a comment for
clarity.

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>