staging: comedi: amplc_pci230: fix the ai/ao cmd->start_arg use for TRIG_INT

This driver trivially validates the cmd->start_arg for all cmd->start_src values
to be 0. For TRIG_INT source, the cmd->start_arg is actually the valid trig_num
that is passed to 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, refactor the ai (*do_cmd) to remove some unnecessary {}.

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>