mt76: mt7921: don't enable beacon filter when IEEE80211_CONF_CHANGE_MONITOR is set

IEEE80211_CONF_CHANGE_MONITOR would be set every time even when vif runs as
sta mode.

If vif runs as sta mode, the beacon filter would be falsely enabled
earlier than the associated state.

That is not the behavior the firmware expects and also breaks the original
logic of sta mode. Thus, we only limit the operation to set the beacon
filter only for monitor mode when IEEE80211_CONF_CHANGE_MONITOR is set.

Fixes: 4ad65a54 ("mt76: mt7921: toggle runtime-pm adding a monitor vif")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>