net: lan966x: Update the PGID used by IPV6 data frames

When enabling the multicast snooping, the forwarding of the IPV6 frames
has it's own forwarding mask.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>