staging: comedi: usbduxfast: fix the cmd->start_arg validation and use

This driver supports cmd->start_src values of TRIG_NOW, TRIG_EXT, and
TRIG_INT. Currently the cmd->start_arg is only validated for TRIG_NOW.

For TRIG_EXT sources, the cmd->start_src is driver specific. For a
TRIG_INT source, the cmd->start_arg is actually the valid trig_num
that is used by the async (*inttrig) callback.

The cmd->start_arg is currently not use in the driver. Just trivially
validate it to be 0 for all cmd->start_src values.

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

For aesthetics, remove some unnecessary comments in the (*do_cmd)
function.

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>