staging: comedi: ni_tiocmd: clarify the cmd->start_arg validation and use

This driver supportes cmd->start_src sources of TRIG_NOW, TRIG_INT, TRIG_OTHER,
and TRIG_EXT. Refactor the (*do_cmdtest) to clarify the trivial validation of
the cmd->start_arg. 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>