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

Currently the async command support in this driver consists of just the
stubbed in functions.

Flesh out the (*do_cmdtest) function for basic support of 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>