drm/i2c: tda998x: index audio port enable config by route type

Rather than searching an array for the audio format (which we control)
implement indexing by route type.  This avoids iterating over the array
in several locations.

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