net: fec: fix multicast filtering hardware setup

Fix hardware setup of multicast address hash:
- Never clear the hardware hash (to avoid packet loss)
- Construct the hash register values in software and then write once
to hardware

Signed-off-by: Rui Sousa <rui.sousa@nxp.com>
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>