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: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>