net: dsa: mt7530: fix handling of LLDP frames

LLDP frames are link-local frames, therefore they must be trapped to the
CPU port. Currently, the MT753X switches treat LLDP frames as regular
multicast frames, therefore flooding them to user ports. To fix this, set
LLDP frames to be trapped to the CPU port(s).

Fixes: b8f126a8 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>