mt76: Remove set but not used variable 'idx'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/mediatek/mt76/dma.c: In function mt76_dma_rx_fill:
drivers/net/wireless/mediatek/mt76/dma.c:377:6: warning: variable idx set but not used [-Wunused-but-set-variable]

It is not used since commit 17f1de56 ("mt76:
add common code shared between multiple chipsets")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: Nzhengbin <zhengbin13@huawei.com>
Signed-off-by: NFelix Fietkau <nbd@nbd.name>

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

@@ -365,7 +365,6 @@ mt76_dma_rx_fill(struct mt76_dev *dev, struct mt76_queue *q)
 	int frames = 0;
 	int len = SKB_WITH_OVERHEAD(q->buf_size);
 	int offset = q->buf_offset;
-	int idx;
 
 	spin_lock_bh(&q->lock);
 
@@ -384,7 +383,7 @@ mt76_dma_rx_fill(struct mt76_dev *dev, struct mt76_queue *q)
 
 		qbuf.addr = addr + offset;
 		qbuf.len = len - offset;
-		idx = mt76_dma_add_buf(dev, q, &qbuf, 1, 0, buf, NULL);
+		mt76_dma_add_buf(dev, q, &qbuf, 1, 0, buf, NULL);
 		frames++;
 	}