r8169: fix incorrect mac address assignment

The original changes brakes MAC address assignment on older chip
versions (see bug report [0]), and it brakes random MAC assignment.

is_valid_ether_addr() requires that its argument is word-aligned.
Add the missing alignment to array mac_addr.

[0] https://bugzilla.kernel.org/show_bug.cgi?id=215087



Fixes: 1c5d09d5 ("ethernet: r8169: use eth_hw_addr_set()")
Reported-by: Richard Herbert <rherbert@sympatico.ca>
Tested-by: Richard Herbert <rherbert@sympatico.ca>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>