staging: comedi: pcl812: cleanup setup_range_channel()

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 does not support analog input channel scanning so the mux and
range need to be set before each (*insn_read) and when advancing to the next
channel in an async command. Instead of storing the last chan/range in the
private data just store the chanspec to determine if the mux and range need
to be changed.

Refactor pcl812_reset() a bit so we can use the helper function to set the
mux and range.

Define the bits in the mux register to remove the magic values.

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>