appletalk: Use ether_addr_copy

Use ether_addr_copy instead of memcpy(a, b, ETH_ALEN) to
save some cycles on arm and powerpc.

Convert struct aarp_entry.hwaddr[6] to hwaddr[ETH_ALEN].

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>