mlxsw: spectrum_ipip: Extract mlxsw_sp_l3addr_is_zero

Extract the logic for determining whether a given IPv4/IPv6 address is
all-zeroes from mlxsw_sp_ipip_tunnel_complete to a separate function.
Make that function public within the module.

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>