[PATCH v2] wifi: mt76: mt7921: fix txpower reporting

[Lucid Duck <lucid_duck@justthetip.ca>]

The mt7921 driver never updates phy->txpower_cur, causing
mt76_get_txpower() to report incorrect values via nl80211.
Users see bogus txpower readings (typically 3 dBm or 67 dBm)
regardless of actual regulatory limits.

Fix this by updating txpower_cur in mt7921_bss_info_changed()
when BSS_CHANGED_TXPOWER is set. Use bss_conf.txpower as the
primary source, with a fallback to the channel's max_reg_power
for cases where bss_conf.txpower is not populated (INT_MIN).

Tested on Alfa AWUS036AXML (MT7921AU), kernel 6.18.6:
  - 2.4GHz ch1:  33 dBm (30 dBm limit + 3 dBm path delta) - PASS
  - 5GHz ch100:  27 dBm (24 dBm limit + 3 dBm path delta) - PASS
  - 6GHz ch5:    15 dBm (12 dBm limit + 3 dBm path delta) - PASS

Signed-off-by: Lucid Duck <lucid_duck@justthetip.ca>
---
 drivers/net/wireless/mediatek/mt76/mt7921/main.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/net/wireless/mediatek/mt76/mt7921/main.c b/drivers/net/wireless/mediatek/mt76/mt7921/main.c
index f37a35b..99fe46d 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7921/main.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7921/main.c
@@ -725,6 +725,25 @@ static void mt7921_bss_info_changed(struct ieee80211_hw *hw,
 	if (changed & BSS_CHANGED_CQM)
 		mt7921_mcu_set_rssimonitor(dev, vif);

+	if (changed & BSS_CHANGED_TXPOWER) {
+		int tx_power = info->txpower;
+
+		/*
+		 * Workaround for kernel 6.18+: bss_conf.txpower may not be
+		 * populated (INT_MIN) even when BSS_CHANGED_TXPOWER is set.
+		 * In this case, use the channel's max regulatory power.
+		 */
+		if (tx_power == INT_MIN || tx_power <= 0) {
+			struct ieee80211_channel *chan = phy->mt76->chandef.chan;
+			if (chan)
+				tx_power = chan->max_reg_power;
+		}
+
+		/* txpower is in dBm, txpower_cur is in 0.5dBm units */
+		if (tx_power > 0 && tx_power < 127)
+			phy->mt76->txpower_cur = tx_power * 2;
+	}
+
 	if (changed & BSS_CHANGED_ASSOC) {
 		mt7921_mcu_sta_update(dev, NULL, vif, true,
 				      MT76_STA_INFO_STATE_ASSOC);
--
2.43.0