ASoC: ak4535: Improve readability for setting mute

The mute/unmute is controled by SMUTE (Soft Mute Control bit):
        0: Normal Operation (Default)
        1: DAC outputs soft-muted

I think this change improves readability.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>