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

This driver supports a cmd->start_src of TRIG_NOW or TRIG_INT. The
cmd->start_arg is trivially validated for both sources to be 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) function so that the cmd->start_arg is used
to check the trig_num instead of the open coded value.

For aesthetics, refactor the (*do_cmd) to use if/else instead if the
switch when handling the cmd->start_src.

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>