ASoC: hdmi-codec: Add a prepare hook

The IEC958 status bit is usually set by the userspace after hw_params
has been called, so in order to use whatever is set by the userspace, we
need to implement the prepare hook. Let's add it to the hdmi_codec_ops,
and mandate that either prepare or hw_params is implemented.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>