net: dsa: slave: Don't propagate flag changes on down slave interfaces

The unbalance of master's promiscuity or allmulti will happen after ifdown
and ifup a slave interface which is in a bridge.

When we ifdown a slave interface , both the 'dsa_slave_close' and
'dsa_slave_change_rx_flags' will clear the master's flags. The flags
of master will be decrease twice.
In the other hand, if we ifup the slave interface again, since the
slave's flags were cleared the 'dsa_slave_open' won't set the master's
flag, only 'dsa_slave_change_rx_flags' that triggered by 'br_add_if'
will set the master's flags. The flags of master is increase once.

Only propagating flag changes when a slave interface is up makes
sure this does not happen. The 'vlan_dev_change_rx_flags' had the
same problem and was fixed, and changes here follows that fix.

Fixes: 91da11f8 ("net: Distributed Switch Architecture protocol support")
Signed-off-by: Rundong Ge <rdong.ge@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>