ASoC: rt5651: Make rt5651_apply_properties() private

The idea behind exporting rt5651_apply_properties(), was for it to be used
on platforms where the platform code may need to add device-properties,
rather then relying only on properties set by the firmware. The platform
code could then call rt5651_apply_properties() after adding properties to
make sure that the codec driver was aware of the new properties.

But this is not necessary, as long as we do all property parsing from
the codec component-driver's probe function (or later) then the machine
driver can attach properties before calling snd_soc_register_card and
calling rt5651_apply_properties() for ordering reasons is not necessary.

This commit makes rt5651_apply_properties() private and adds 2 comments
documenting that all property parsing must be done from the codec
component-driver's probe function.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mark Brown <broonie@kernel.org>