staging: comedi: usbduxsigma: fix the ai/ao cmd->start_arg use for TRIG_INT

For both ai and ao commands 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) functions so that the cmd->start_arg is used
to check the trig_num instead of the open coded values.

For aesthetics, remove some unnecessary comments in the (*do_cmd)
functions.

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>