staging: comedi: pcl816: cleanup setup_channel_list()

Move this function to remove the need for the forward declaration. Rename it
so it has namespace associated with the driver. Remove the unnecessary
comedi_subdevice parameter from the function.

The hardware has per-channel programmable gain. This function first sets the
range for each channel then sets the mux register to automatically scan the
channels.

Remove the need for the 'ai_act_chanlist' member in the private data. It is
only used to set the first/last channel to scan.

Introduce a couple helper functions to set the range for a channel and to set
the first/last channels to scan.

Tidy up the range and mux register defines.

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>