ASoC: SOF: expose a couple of functions

Expose the mixer_to_ipc() and ipc_to_mixer() functions for reuse in
IPC4.

Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20220426171743.171061-6-ranjani.sridharan@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>