mlxsw: spectrum: Fix incorrect reuse of MID entries

In the device, a MID entry represents a group of local ports, which can
later be bound to a MDB entry.

The lookup of an existing MID entry is currently done using the provided
MC MAC address and VID, from the Linux bridge. However, this can result
in an incorrect reuse of the same MID index in different VLAN-unaware
bridges (same IP MC group and VID 0).

Fix this by performing the lookup based on FID instead of VID, which is
unique across different bridges.

Fixes: 3a49b4fd ("mlxsw: Adding layer 2 multicast support")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>