Disable audio codec wait-for-zero-crossing volume change mode.

This may explain (now unreproduceable) issue where UI volume changes weren't always changing the gain on the audio codec.

firmware/common/wm8731.hpp

@@ -374,7 +374,7 @@ public:
 
 		write(LeftHeadphoneOut {
 			.lhpvol = static_cast<reg_t>(n),
-			.lzcen = 1,
+			.lzcen = 0,
 			.lrhpboth = 1,
 			.reserved0 = 0,
 		});