drm/amdgpu: fix crash in acp_hw_fini

On CZ/ST systems with AZ rather than ACP audio, we need to bail
early in hw_fini since there is nothing to do.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=98276



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org