mt76: mt7915: fix tx skb dma unmap

mt76: mt7915: fix tx skb dma unmap

BugLink: https://bugs.launchpad.net/bugs/1929455
[ Upstream commit 7dcf3c04f0aca746517a77433b33d40868ca4749 ]

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

Reported-by: Ben Greear <greearb@candelatech.com>
Fixes: 27d5c528a7ca ("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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>