net: dsa: Introduce dsa_is_port_initialized

To avoid race conditions when using the ds->ports[] array,
we need to check if the accessed port has been initialized.
Introduce and use helper function dsa_is_port_initialized
for that purpose and use it where needed.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>