ASoC: WM8804: Power down the PLL correctly

The PLL is disabled when the corresponding bit is set not the other
way around.  This commit depends on my other commit with Subject
"ASoC: WM8804: Refactor set_pll code to avoid GCC warnings".

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>