wifi: mac80211: apply mcast rate only if interface is up

If the interface isn't enabled, don't apply multicast
rate changes immediately.

Reported-by:  <syzbot+de87c09cc7b964ea2e23@syzkaller.appspotmail.com>
Link: https://msgid.link/20240515133410.d6cffe5756cc.I47b624a317e62bdb4609ff7fa79403c0c444d32d@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>