lio: Avoid out-of-bounds read or write MAC address

While here, replace loop copying the MAC address with memcpy() for
better readability.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54177

(cherry picked from commit 094626d3a5009a56bf1b763dbdfc681ce371dc99)