staging: comedi: ni_mio_common: factor out chanlist checking from (*do_cmdtest)

Step 5 of the (*do_cmdtest) validates that the cmd->chanlist is compatible
with the hardware.

Remove the unnecessary step 5 code in the analog output async command support.

For aesthetics, factor out the step 5 code for the "cdio" async command support.
Tidy up the factored out code.

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>