drm/tda998x: BUG() on invalid audio format

Suppress warning of unused-variables by adding a BUG()+return for invalid
audio-formats.

Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>