mt76x2: configure rx filter based on monitor mode setting

Due to an unrelated issue, the MT_RX_FILTR_CFG_PROMISC flag is currently
unset, which means that monitor mode is unconditionally enabled.
Toggle this flag based on the mac80211 monitor mode setting instead

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>