nl80211: multicast_to_unicast can be changed while IFF_UP

There is no need to prevent toggling multicast_to_unicast while
interface is already up. This change simplifies reconfiguration
from hostapd.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>