ASoC: adav80x: Use devm_kzalloc()

Use devm_kzalloc() to allocate the device state struct. Saves use from having to
free it manually on the error path and in the remove callback.

Now that the adav80x_bus_probe() function is only a call to
snd_soc_unregister_codec() also inline that.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>