mt76: mt7615: fix monitor injection of beacon frames

When injecting beacon frames via monitor interface, they must not be sent to
the beacon hardware queue, because they don't follow normal hardware beacon tx
rules.
Fix sending them by adding a flag to mt7615_mac_write_txwi that selects the
beacon queue for tx, and use it only from mt7615_mcu_set_bcn.

Signed-off-by: Felix Fietkau <nbd@nbd.name>