net: macb: support of_get_mac_address new ERR_PTR error

There was NVMEM support added directly to of_get_mac_address, and it uses
nvmem_get_mac_address under the hood, so we can remove it. As
of_get_mac_address can now return ERR_PTR encoded error values, adjust to
that as well.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>