mt76: mt7915: fix bit fields for HT rate idx

The bit fields of tx rate idx should be 6 bits, otherwise it might be
incorrect in HT mode.
For VHT/HE rates, only 4 bits are actually used by rate idx, the other
2 bits are used for other functions.

Fixes: c31d94af ("mt76: mt7915: fix tx rate related fields in tx descriptor")
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/mac.h

@@ -280,7 +280,8 @@ enum tx_mcu_port_q_idx {
 #define MT_TX_RATE_MODE			GENMASK(9, 6)
 #define MT_TX_RATE_SU_EXT_TONE		BIT(5)
 #define MT_TX_RATE_DCM			BIT(4)
-#define MT_TX_RATE_IDX			GENMASK(3, 0)
+/* VHT/HE only use bits 0-3 */
+#define MT_TX_RATE_IDX			GENMASK(5, 0)
 
 #define MT_TXP_MAX_BUF_NUM		6