net: l2: ethernet: arp: Fix ARP probe target HW address matching

According to RFC3927 and RFC5227, an ARP probe target HW address should
be set to all-zeroes:

"The 'target hardware address' field is ignored and
SHOULD be set to all zeroes."

Hence, we should allow the ARP probes to have all-zeroes target HW
address as well.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>