wifi: mt76: fix per-band IEEE80211_CONF_MONITOR flag comparison

Use the correct ieee80211_conf of each band for IEEE80211_CONF_MONITOR
comparison.

Fixes: 24e69f6b ("mt76: fix monitor rx FCS error in DFS channel")
Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>