mt76: mt7915: fix the nss setting in bitrates

without this change, the fixed MCS only supports 1 Nss.

Fixes: 70fd1333 ("mt76: mt7915: rework .set_bitrate_mask() to support more options")
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/mcu.c

@@ -1427,9 +1427,12 @@ mt7915_mcu_add_rate_ctrl_fixed(struct mt7915_dev *dev,
 			phy.sgi |= gi << (i << (_he));				\
 			phy.he_ltf |= mask->control[band].he_ltf << (i << (_he));\
 		}								\
-		for (i = 0; i < ARRAY_SIZE(mask->control[band]._mcs); i++) 	\
+		for (i = 0; i < ARRAY_SIZE(mask->control[band]._mcs); i++) {	\
+			if (!mask->control[band]._mcs[i])			\
+				continue;					\
 			nrates += hweight16(mask->control[band]._mcs[i]);	\
-		phy.mcs = ffs(mask->control[band]._mcs[0]) - 1;			\
+			phy.mcs = ffs(mask->control[band]._mcs[i]) - 1;		\
+		}								\
 	} while (0)
 
 	if (sta->he_cap.has_he) {