ASoC: wm8960: Fix WM8960_SYSCLK_PLL mode

With the introduction of WM8960_SYSCLK_AUTO mode, WM8960_SYSCLK_PLL mode was
made unusable.  Ensure we're not PLL mode before trying to use MCLK.

Fixes: 3176bf2d

 ("ASoC: wm8960: update pll and clock setting function")
Signed-off-by: Stuart Henderson <stuart.henderson@cirrus.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>