audioconvert: add a invalid value for scale

The invalid value here is zero, this will help avoid inadvertant parameter
updates and brings this on par with rest of the volume ramp parameters

spa/include/spa/param/audio/raw.h

@@ -262,6 +262,7 @@ enum spa_audio_channel {
 };
 
 enum spa_audio_volume_ramp_scale {
+	SPA_AUDIO_VOLUME_RAMP_INVALID,
 	SPA_AUDIO_VOLUME_RAMP_LINEAR,
 	SPA_AUDIO_VOLUME_RAMP_CUBIC,
 };