net: dsa: setup and teardown default CPU port

The dsa_dst_parse function called just before dsa_dst_apply does not
parse the tree but does only one thing: it assigns the default CPU port
to dst->cpu_dp and to each user ports.

This patch simplifies this by calling a dsa_tree_setup_default_cpu
function at the beginning of dsa_dst_apply directly.

A dsa_port_is_user helper is added for convenience.

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