staging: comedi: das6402: add (*do_cmd) for AI async commands

Flesh out the (*do_cmd) function to support timed analog input
acquisitions.

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>