wifi: mt76: mt7921: fix potential tx_retries underflow

When FIELD_GET returns 0 for the retry count, subtracting 1 causes
an unsigned integer underflow, resulting in tx_retries becoming a
very large value (0xFFFFFFFF for u32).

Fix by checking if count is non-zero before subtracting 1.

Fixes: 9aecfa754c ("wifi: mt76: mt7921e: report tx retries/failed counts in tx free event")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://patch.msgid.link/20260605113306.3485554-2-ryder.lee@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7921/mac.c

@@ -531,8 +531,9 @@ static void mt7921_mac_tx_free(struct mt792x_dev *dev, void *data, int len)
 		stat = FIELD_GET(MT_TX_FREE_STATUS, info);
 
 		if (wcid) {
-			wcid->stats.tx_retries +=
-				FIELD_GET(MT_TX_FREE_COUNT, info) - 1;
+			u32 count = FIELD_GET(MT_TX_FREE_COUNT, info);
+
+			wcid->stats.tx_retries += count ? count - 1 : 0;
 			wcid->stats.tx_failed += !!stat;
 		}