linuxkpi: use canonical tests for is_{zero,broadcast}_ether_addr

They are functionally equivalent, but the updated form mirrors the tests
in sys/net/ethernet.h and avoids confusion.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21037

(cherry picked from commit a1295b24842d209e6bf93a4823193d56ad2db064)