ASoC: SOF: sof-client: Convert sof_client_dev_to_sof_dev into function

Remove the sof_client_dev_to_sof_dev() from the header file and add it as
a function in sof-client.c to avoid it's use by client drivers.

At the same time mark the sdev and list of sof_client_dev as restricted
fro core use only.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Message-ID: <20250829102510.15159-2-peter.ujfalusi@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>