ASoC: WM8994: Improve playback robustness

On WM8994 revision D and earlier ensure proper playback robustness
as some rare use cases can trigger issues.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org