mt76: fix potential DMA mapping leak

With buf uninitialized in mt76_dma_tx_queue_skb_raw, its field skip_unmap
could potentially inherit a non-zero value from stack garbage.
If this happens, it will cause DMA mappings for MCU command frames to not be
unmapped after completion

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

drivers/net/wireless/mediatek/mt76/dma.c

@@ -318,7 +318,7 @@ static int
 mt76_dma_tx_queue_skb_raw(struct mt76_dev *dev, struct mt76_queue *q,
 			  struct sk_buff *skb, u32 tx_info)
 {
-	struct mt76_queue_buf buf;
+	struct mt76_queue_buf buf = {};
 	dma_addr_t addr;
 
 	if (q->queued + 1 >= q->ndesc - 1)