wifi: mt76: mt7996: fix endianness of MT_TXD6_TX_RATE

To avoid sparse warning:
sparse: warning: invalid assignment: |=
sparse:    left side has type restricted __le32
sparse:    right side has type unsigned lon

Fixes: 15ee62e7 ("wifi: mt76: mt7996: enable BSS_CHANGED_BASIC_RATES support")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/16fa938373e3b145cb07a2c98d2428fea2abadba.1682285873.git.ryder.lee@mediatek.com

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

@@ -1088,7 +1088,7 @@ void mt7996_mac_write_txwi(struct mt7996_dev *dev, __le32 *txwi,
 		else if (beacon && mvif->beacon_rates_idx)
 			idx = mvif->beacon_rates_idx;
 
-		txwi[6] |= FIELD_PREP(MT_TXD6_TX_RATE, idx);
+		txwi[6] |= cpu_to_le32(FIELD_PREP(MT_TXD6_TX_RATE, idx));
 		txwi[3] |= cpu_to_le32(MT_TXD3_BA_DISABLE);
 	}
 }