ethernet: Convert mac address uses of 6 to ETH_ALEN

Use the normal #define to help grep find mac addresses
and ensure that addresses are aligned.

pasemi.h has an unaligned access to mac_addr, unchanged
for now.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Olof Johansson <olof@lixom.net> # pasemi_mac pieces
Signed-off-by: David S. Miller <davem@davemloft.net>