mt76: mt7663s: make all of packets 4-bytes aligned in sdio tx aggregation

Each packet should be padded with the additional zero to become 4-bytes
alignment in sdio tx aggregation.

Fixes: 1522ff73 ("mt76: mt7663s: introduce sdio tx aggregation")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>