mlxsw: spectrum_router: Extract mlxsw_sp_ipip_entry_ol_up_event()

The piece of logic to promote decap route, if any, is useful for generic
tunnel updates, not just for handling of NETDEV_UP events on tunnel
interfaces. Extract it to a separate function.

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