use proper test for all-zero or all-fs mac (one byte isn't enough)

Reference: /n/atom/patch/applied2013/usbsmsc
Date: Tue Jun 18 21:42:37 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/usb/ether/smsc.c	Tue Jun 18 21:42:12 2013
+++ /sys/src/cmd/usb/ether/smsc.c	Tue Jun 18 21:42:12 2013
@@ -205,19 +205,19 @@
 	return 0;
 }
 
+uchar	nilea[Eaddrlen];
+uchar	fsea[Eaddrlen] = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff, };
+
 static int
 getmac(Dev *d, uchar buf[])
 {
-	int i;
 	uchar ea[Eaddrlen];
 
 	if(eepromr(d, MACoffset, ea, Eaddrlen) < 0)
 		return -1;
-	for(i = 0; i < Eaddrlen; i++)
-		if(ea[i] != 0 && ea[i] != 0xFF){
-			memmove(buf, ea, Eaddrlen);
-			break;
-		}
+	if(memcmp(ea, nilea, Eaddrlen) != 0)
+	if(memcmp(ea, fsea, Eaddrlen) != 0)
+		memmove(buf, ea, Eaddrlen);
 	return Eaddrlen;
 }