mt76x0: do not overwrite other MT_BBP(AGC, 8) fields

MT_BBP(AGC, 8) register has values depend on band in
mt76x0_bbp_switch_tab, so we should not overwrite other fields
than MT_BBP_AGC_GAIN when setting gain.

This can fix performance issues when connecting to 2.4GHz AP.

Fixes: 4636a254 ("mt76x0: phy: align channel gain logic to mt76x2 one")
Acked-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt76x0/phy.c

@@ -715,10 +715,8 @@ static void mt76x0_phy_temp_sensor(struct mt76x02_dev *dev)
 static void mt76x0_phy_set_gain_val(struct mt76x02_dev *dev)
 {
 	u8 gain = dev->cal.agc_gain_cur[0] - dev->cal.agc_gain_adjust;
-	u32 val = 0x122c << 16 | 0xf2;
 
-	mt76_wr(dev, MT_BBP(AGC, 8),
-		val | FIELD_PREP(MT_BBP_AGC_GAIN, gain));
+	mt76_rmw_field(dev, MT_BBP(AGC, 8), MT_BBP_AGC_GAIN, gain);
 
 	if ((dev->mt76.chandef.chan->flags & IEEE80211_CHAN_RADAR) &&
 	    !is_mt7630(dev))