staging: comedi: das1800: refactor program_chanlist()

Refactor this function so it can be used to program the chanlist for
the analog input (*do_cmd) and (*insn_read).

Rename the function so it has namespace associated with the driver.

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>