ASoC: mediatek: use snd_soc_dai_get_drvdata() to get the private data

Reduce the boilerplate code to retrieve the private data.
No functional change intended.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Garlic Tseng <garlic.tseng@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>