ASoC: wm8994: Lower AIFnCLK divisor when dropping to 32kHz

When lowering SYSCLK to 50kHz for accessory detection also lower the
AIFnCLK divisor to normalise the clocking configuration within the
device. This will not disrupt audio as we cannot support active audio
with such a low SYSCLK.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>