ASoC: max9768: Fix up review comments in max9768 driver

The Mute Switch should be called Playback Switch to match the volume
and the regmap core has been updated so we don't need to mark all the
registers as non-volatile.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>