ASoC: sun8i-codec: Add a comment on the LRCK inversion

The current code might be a bit intriguing without having experienced the
issue before, and might come up as a mistake.

Make explicit what's going on by adding a comment.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>