net: lan966x: Fix IGMP snooping when frames have vlan tag

In case an IGMP frame has a vlan tag, then the function
lan966x_hw_offload couldn't figure out that is a IGMP frame. Therefore
the SW thinks that the frame was already forward by the HW which is not
true.
Extend lan966x_hw_offload to pop the vlan tag if are any and then check
for IGMP frames.

Fixes: 47aeea0d ("net: lan966x: Implement the callback SWITCHDEV_ATTR_ID_BRIDGE_MC_DISABLED ")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>