staging: comedi: amplc_dio200_common: fix the cmd->start_arg use

This driver supports two cmd->start_src values, TRIG_NOW and TRIG_INT. TRIG_NOW
sources should always have an arg of 0. For TRIG_INT sources, the cmd->start_arg
is actually the valid trig_num that is passed to the async (*inttrig) callback.
This driver trivially validates the arg for both sources to be 0.

Refactor the (*inttrig) 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 of 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>