wifi: mt76: mt7925: 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: c948b5d ("wifi: mt76: mt7925: add Mediatek Wi-Fi7 driver for mt7925 chips")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://patch.msgid.link/20260605113306.3485554-3-ryder.lee@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit 1e1fd84571e62a2961cea44c053340ec5c99b2cb)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: ryderlee1110

drivers/net/wireless/mediatek/mt76/mt7925/mac.c

@@ -1150,8 +1150,9 @@ mt7925_mac_tx_free(struct mt792x_dev *dev, void *data, int len)
 
 		if (info & MT_TXFREE_INFO_HEADER) {
 			if (wcid) {
-				wcid->stats.tx_retries +=
-					FIELD_GET(MT_TXFREE_INFO_COUNT, info) - 1;
+				u32 count = FIELD_GET(MT_TXFREE_INFO_COUNT, info);
+
+				wcid->stats.tx_retries += count ? count - 1 : 0;
 				wcid->stats.tx_failed +=
 					!!FIELD_GET(MT_TXFREE_INFO_STAT, info);
 			}