Fix the uninitialized variable warning in mt7996_mac_tx_free.

Fixes: 2461599f ("wifi: mt76: mt7996: get tx_retries and tx_failed from txfree")
Signed-off-by: Yi-Chia Hsieh <yi-chia.hsieh@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>