net: dsa: return -EOPNOTSUPP when driver does not implement .port_lag_join

The DSA core has a layered structure, and even though we end up
returning 0 (success) to user space when setting a bonding/team upper
that can't be offloaded, some parts of the framework actually need to
know that we couldn't offload that.

For example, if dsa_switch_lag_join returns 0 as it currently does,
dsa_port_lag_join has no way to tell a successful offload from a
software fallback, and it will call dsa_port_bridge_join afterwards.
Then we'll think we're offloading the bridge master of the LAG, when in
fact we're not even offloading the LAG. In turn, this will make us set
skb->offload_fwd_mark = true, which is incorrect and the bridge doesn't
like it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>