ASoC: SOF: ipc4: Add define for the outbox window index

ASoC: SOF: ipc4: Add define for the outbox window index

Instead of using the index number directly, add a define for the outbox
window index.
It is always window 1 with IPC4.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20220909114332.31393-4-peter.ujfalusi@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>