bridge: Use ether_addr_copy and ETH_ALEN

Convert the more obvious uses of memcpy to ether_addr_copy.

There are still uses of memcpy that could be converted but
these addresses are __aligned(2).

Convert a couple uses of 6 in gr_private.h to ETH_ALEN.

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