staging: comedi: ni_tiocmd: change mistaken use of start_src for start_arg

This fixes a bug in function ni_tio_input_inttrig().  The trigger number
should be compared to cmd->start_arg, not cmd->start_src.

Fixes: 6a760394 ("staging: comedi: ni_tiocmd: clarify the cmd->start_arg validation and use")
Cc: <stable@vger.kernel.org> # 3.17+
Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>