ieee802154: Fix generation of random EUI-64 addresses.

Currently, ieee802154_random_extended_addr() has a 50% chance of
generating a group (multicast) address, while this function is used
for generating station addresses (which can't be group addresses)
for interfaces that don't have a hardware-provided address.

Also, in case get_random_bytes() generates the EUI-64 address
00:00:00:00:00:00:00:00 (extremely unlikely), which is an invalid
address, ieee802154_random_extended_addr() reacts by changing it
to 01:00:00:00:00:00:00:00, which is an invalid station address as
well, as it is a group address.

This patch changes the address generation procedure to grab eight
random bytes, treat that as an EUI-64, and then clear the Group
address bit and set the Locally Administered bit, which is in
line with how eth_random_addr() generates random EUI-48s.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>