device property: move mac addr helpers to eth.c

Move the mac address helpers out, eth.c already contains
a bunch of similar helpers.

Suggested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>