net: davinci_emac: Get device dm816x MAC address using the cpsw code

At least on dm81xx, we can get the davinci_emac MAC address the same
way as on am33xx cpsw.

Let's also use ether_addr_copy() for davinci_emac while at it.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>