ASoC: es8316: fix redundant codes of clock

This patch removes redundant null checks for optional MCLK clock.
And fix DT binding document for changing clock property to optional
from required.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
Link: https://lore.kernel.org/r/20190907163653.9382-1-katsuhiro@katsuster.net
Signed-off-by: Mark Brown <broonie@kernel.org>