wifi: mt76: connac: set fixed_bw bit in TX descriptor for fixed rate frames

Always set the fixed_bw bitfield for fixed rate frames to keep it being
sent with specific bandwidth. Without this change, the bw of fixed rate
frames will still be decided by hardware.
Reported-by: Chank Chen <chank.chen@mediatek.com>
Signed-off-by: Howard Hsu <howard-yh.hsu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt76_connac3_mac.h

@@ -239,7 +239,8 @@ enum tx_mgnt_type {
 
 #define MT_TXD6_TX_SRC			GENMASK(31, 30)
 #define MT_TXD6_VTA			BIT(28)
-#define MT_TXD6_BW			GENMASK(25, 22)
+#define MT_TXD6_FIXED_BW		BIT(25)
+#define MT_TXD6_BW			GENMASK(24, 22)
 #define MT_TXD6_TX_RATE			GENMASK(21, 16)
 #define MT_TXD6_TIMESTAMP_OFS_EN	BIT(15)
 #define MT_TXD6_TIMESTAMP_OFS_IDX	GENMASK(14, 10)

drivers/net/wireless/mediatek/mt76/mt7996/mac.c

@@ -943,7 +943,8 @@ void mt7996_mac_write_txwi(struct mt7996_dev *dev, __le32 *txwi,
 				idx = mvif->basic_rates_idx;
 		}
 
-		txwi[6] |= cpu_to_le32(FIELD_PREP(MT_TXD6_TX_RATE, idx));
+		val = FIELD_PREP(MT_TXD6_TX_RATE, idx) | MT_TXD6_FIXED_BW;
+		txwi[6] |= cpu_to_le32(val);
 		txwi[3] |= cpu_to_le32(MT_TXD3_BA_DISABLE);
 	}
 }