Fix regression introduced by gf3b631668b

I made a stupid when simplifying the bit-twiddling, and accidentally reversed
two of the bytes which completely broke MAC address name resolution.

Bug: 10344
Change-Id: I0720755fb290423150e4d84da9d45cb0b76341e4
Reviewed-on: https://code.wireshark.org/review/3522
Reviewed-by: Michael Mann <mmann78@netscape.net>