mlxsw: spectrum: Move handling of tunnel events to router code

The events related to IPIP tunnels are handled by the router code. Move the
handling from the central dispatcher in spectrum.c to the new notifier
handler in the router module.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>