net: dsa: get tree before parsing ports

We will need a reference to the dsa_switch_tree when parsing a CPU port,
so fetch it right after parsing the member and before parsing ports.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>