emac: aggregation of v1-2 PLB errors for IER register

Aggreagation of version 1-2 because of version 1 can hit
PLB errors too. If it's not set so we missing events for PLB bits
and driver can't process those interrupts.

Signed-off-by: Ivan Mikhaylov <ivan@ru.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>