ASoC: stac9766 - set reg_cache_default to stac9766_reg

Looks like this is missing during multi-component conversion.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>