fix unused variable warning

Reference: /n/atom/patch/applied/usbaudiowarn
Date: Sat Apr 19 16:14:11 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/usb/audio/audiosub.c	Sat Apr 19 16:14:11 2014
+++ /sys/src/cmd/usb/audio/audiosub.c	Sat Apr 19 16:14:11 2014
@@ -58,7 +58,6 @@
 	byte *p;
 	int class, subclass;
 	Audioalt *aa;
-	char *hd;
 
 	class = Class(dd->iface->csp);
 	subclass = Subclass(dd->iface->csp);