net: dsa: remove workarounds for changing master promisc/allmulti only while up

Lennert Buytenhek explains in commit df02c6ff ("dsa: fix master
interface allmulti/promisc handling"), dated Nov 2008, that changing the
promiscuity of interfaces that are down (here the master) is broken.

This fact regarding promisc/allmulti has changed since commit
b6c40d68 ("net: only invoke dev->change_rx_flags when device is UP")
by Vlad Yasevich, dated Nov 2013.

Therefore, DSA now has unnecessary complexity to handle master state
transitions from down to up. In fact, syncing the unicast and multicast
addresses can happen completely asynchronously to the administrative
state changes.

This change reduces that complexity by effectively fully reverting
commit df02c6ff ("dsa: fix master interface allmulti/promisc
handling").

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