drm/i2c: tda998x: store audio port enable in settings

Store the audio port enable register in the audio settings structure,
which can never be zero for a valid audio configuration.  Use this to
signal whether we have audio configured, rather than AFMT_UNUSED.

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