drm/i2c: tda998x: clean up tda998x_configure_audio()

tda998x_configure_audio() is called via some paths where an error
return is meaningless, and as a result of moving the audio routing
code, this function no longer returns any errors, so let's make it
void. We can also make tda998x_write_aif() return void as well.

tda998x_configure_audio() also only ever needs to write the current
audio settings, so simplify the code in tda998x_audio_hw_params()
so that can happen.

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