mt76: mt7663: fix DMA unmap length

Fix DMA unmap length for mt7663e devices in mt7615_txp_skb_unmap_hw

Fixes: f40ac0f3 ("mt76: mt7615: introduce mt7663e support")
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Co-developed-by: Soul Huang <soul.huang@mediatek.com>
Signed-off-by: Soul Huang <soul.huang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>