dm: sound: wm8994: Pass private data to internal functions

At present the driver-private data is global. To allow this code to be
used with driver model, change it to pass the data down to each function.
Use the name 'priv' consistently throughout.

Also rename wm8994_update_bits() to wm8994_bic_or() which is more
descriptive and shorter, thus breaking fewer lines with the parameter
addition.
Signed-off-by: NSimon Glass <sjg@chromium.org>