In some cases like NanoPI R1, its second USB ethernet
RTL8152 (chip version URE_CHIP_VER_4C10) doesn't
have hardwired MAC address, in other words, it is all zeros.
This commit fixes it by setting random MAC address
when MAC address is all zeros.

Reviewed by: kevlo
Differential Revision: https://reviews.freebsd.org/D19856