staging: comedi: usbdux: fix the cmd->start_arg use

For all cmd->start_src values this driver trivially validates that the
cmd->start_arg is 0.

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

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

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>