wifi: mt76: mt7915: fix potential tx_retries underflow

mainline inclusion
from mainline-v7.2-rc1
category: bugfix

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: 943e4fb ("wifi: mt76: mt7915: report tx retries/failed counts for non-WED path")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://patch.msgid.link/20260605113306.3485554-1-ryder.lee@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit 05e72b6167970043348bfbe8f72a3b67a38a9f1c)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: ryderlee1110

drivers/net/wireless/mediatek/mt76/mt7915/mac.c

@@ -916,16 +916,16 @@ mt7915_mac_tx_free(struct mt7915_dev *dev, void *data, int len)
 		}
 
 		if (!mtk_wed_device_active(&mdev->mmio.wed) && wcid) {
-			u32 tx_retries = 0, tx_failed = 0;
+			u32 tx_retries = 0, tx_failed = 0, count;
 
 			if (v3 && (info & MT_TX_FREE_MPDU_HEADER_V3)) {
-				tx_retries =
-					FIELD_GET(MT_TX_FREE_COUNT_V3, info) - 1;
+				count = FIELD_GET(MT_TX_FREE_COUNT_V3, info);
+				tx_retries = count ? count - 1 : 0;
 				tx_failed = tx_retries +
 					!!FIELD_GET(MT_TX_FREE_STAT_V3, info);
 			} else if (!v3 && (info & MT_TX_FREE_MPDU_HEADER)) {
-				tx_retries =
-					FIELD_GET(MT_TX_FREE_COUNT, info) - 1;
+				count = FIELD_GET(MT_TX_FREE_COUNT, info);
+				tx_retries = count ? count - 1 : 0;
 				tx_failed = tx_retries +
 					!!FIELD_GET(MT_TX_FREE_STAT, info);
 			}