greybus: audio: Rename Audio class and remove the unused one

There should be a single class macro for Audio and two protocol macros.
Rename class with value 0x12 as GREYBUS_CLASS_AUDIO and remove the other
unused class GREYBUS_CLASS_AUDIO_DATA.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>