powerpc: tsi108: 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.

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