MFD: twl4030-codec: Rename internals from codec to audio

In preparation of renaming the driver from twl4030-codec
to twl4030-audio, first do some clean ups in the driver,
which does not cause any problems outside.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>