Fix MAC address access in 3c507, ibmlana, pcnet32 and libertas

Commit f001fde5eadd915f4858d22ed70d7040f48767cf changed
net_device.dev_addr from a 32-byte array to a pointer.

I found 4 ethernet drivers which rely on sizeof(dev_addr), which are now
only copying 4 bytes of the address information on 32bit systems.

Fix them to use ETH_ALEN.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Reviewed-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>