ASoC: WM8994: Ensure MICBIAS is provided with a clock

The patch 'ASoC: WM8994: Improve Playback Robustness' did not handle
this case properly.

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>