net: dsa: get port type at parse time

Assign a port's type at parsed time instead of waiting for the tree to
be completed.

Because this is now done earlier, we can use the port's type in
dsa_port_is_* helpers instead of digging again in topology description.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>