staging: comedi: s626: clarify the cmd->start_arg validation and use

This driver supports a cmd->start_src of TRIG_NOW, TRIG_INT or TRIG_EXT.

Clarify the cmd->start_arg validation in Step 3 of the (*do_cmdtest).

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) 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>