mlxsw: spectrum: Rename IGMP trap group

The IGMP trap group will be used for MLD traps in the next patch, so
rename it to "MC_SNOOPING" which is more appropriate.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>