net: dsa: lan9303: Set up trapping of IGMP to CPU port

IGMP packets should be trapped to the CPU port. The SW bridge knows
whether to forward to other ports.

With "IGMP snooping for local traffic" merged, IGMP trapping is also
required for stable IGMPv2 operation.

LAN9303 does not trap IGMP packets by default.

Enable IGMP trapping in lan9303_setup.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>