drm/i2c: tda998x: make the audio code more readable

This patch adds a definition of the values of the MUX_AP register and
simplifies the macro's defining the fields of the AIP_CLKSEL register.
This makes the format specific audio init sequence more readable.

Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>