mt76: mt7915: fix tx skb dma unmap

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