net: dsa: rename teardown_default_cpu to teardown_cpu_ports

There is nothing specific to having a default CPU port to what
dsa_tree_teardown_default_cpu() does. Even with multiple CPU ports,
it would do the same thing: iterate through the ports of this switch
tree and reset the ->cpu_dp pointer to NULL. So rename it accordingly.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>