mt76: mt7921: fix endianness in mt7921_mcu_tx_done_event

Fix endianness in mt7921_mcu_tx_done_event event reported by the
firmware.

Fixes: 3cce2b98 ("mt76: mt7921: introduce mac tx done handling")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>