ASoC: codecs: wm8904: Fix register cache incoherency

The multi-component patch(commit f0fba2ad) moved the allocation of the
register cache from the driver to the ASoC core. Most drivers where adjusted to
this, but the wm8904 driver still uses its own register cache for its
private functions, while functions from the ASoC core use the generic cache.
Thus we end up with two from each other incoherent caches, which can lead to
undefined behaviour.
This patch fixes the issue by changing the wm8904 driver to use the
generic register cache in its private functions.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Ian Lartey <ian@opensource.wolfsonmicro.com>
Cc: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org (for 2.6.37 only)