Documentation: networking: dsa: mention that the master is brought up automatically

Since commit 9d5ef190 ("net: dsa: automatically bring up DSA master
when opening user port"), DSA manages the administrative status of the
host port automatically. Update the configuration steps to reflect this.

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