drm/i915/audio: do not mess with audio registers if port is invalid

We should no longer enter the codec enable/disable functions in question
with port A anyway, but to err on the safe side, keep the warnings. Just
bail out early without messing with the registers.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>