mt76: mt7615: fix tx skb dma unmap

[ Upstream commit ebee7885 ]

The first pointer in the txp needs to be unmapped as well, otherwise it will
leak DMA mapping entries

Fixes: 27d5c528 ("mt76: fix double DMA unmap of the first buffer on 7615/7915")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>