net: ethernet: support of_get_mac_address new ERR_PTR error

There was NVMEM support added to of_get_mac_address, so it could now
return ERR_PTR encoded error values, so we need to adjust all current
users of of_get_mac_address to this new fact.

While at it, remove superfluous is_valid_ether_addr as the MAC address
returned from of_get_mac_address is always valid and checked by
is_valid_ether_addr anyway.

Fixes: d01f449c ("of_net: add NVMEM support to of_get_mac_address")
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>