staging: comedi: dt282x: fix the ao cmd->start_arg use for TRIG_INT

This driver only supports a cmd->start_src of TRIG_INT for ao commands. The
cmd->start_arg is trivially validated to be 0. For 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>