drm/i2c: tda998x: move audio routing configuration

Move the mux and clocking selection out of tda998x_configure_audio()
into the parent functions, so we can validate this when parameters
are set outside of the audio mutex.

Tested-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>