rtl8xxxu: Don't printk raw binary if serial number is not burned in.

I assume that a blank efuse comes with all ones, thus I did not bother
recognizing other possible junk values.  This matches 100% of dongles
I've seen (a single Gembird 8192eu).
Signed-off-by: NAdam Borowski <kilobyte@angband.pl>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_8192e.c

@@ -614,7 +614,10 @@ static int rtl8192eu_parse_efuse(struct rtl8xxxu_priv *priv)
 
 	dev_info(&priv->udev->dev, "Vendor: %.7s\n", efuse->vendor_name);
 	dev_info(&priv->udev->dev, "Product: %.11s\n", efuse->device_name);
-	dev_info(&priv->udev->dev, "Serial: %.11s\n", efuse->serial);
+	if (memchr_inv(efuse->serial, 0xff, 11))
+		dev_info(&priv->udev->dev, "Serial: %.11s\n", efuse->serial);
+	else
+		dev_info(&priv->udev->dev, "Serial not available.\n");
 
 	if (rtl8xxxu_debug & RTL8XXXU_DEBUG_EFUSE) {
 		unsigned char *raw = priv->efuse_wifi.raw;