staging: comedi: cb_pcidas64: validate the ai/ao cmd->start_arg

The cmd->start_arg should be trivially validated in Step 3 of the
(*do_cmdtest) functions. Add the necessary code.

For the analog output, a cmd->start_src of TRIG_NOW and TRIG_INT is
supported. For both sources the cmd->start_arg is validated to be 0.
For a TRIG_INT source, the cmd->start_Arg is actually the valid trig_num
that is passed to the async (*inttrig) callback.

Refactor the (*inttrig) function so that the cmd->start_arg is used to
check the trig_num instead of the open coded value.

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>