mt76: mt7915: fix a signedness bug in mt7915_mcu_apply_tx_dpd()

"idx" needs to be signed for the error handling to work.

Fixes: 495184ac ("mt76: mt7915: add support for applying pre-calibration data")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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

@@ -3448,8 +3448,9 @@ int mt7915_mcu_apply_tx_dpd(struct mt7915_phy *phy)
 {
 	struct mt7915_dev *dev = phy->dev;
 	struct cfg80211_chan_def *chandef = &phy->mt76->chandef;
-	u16 total = 2, idx, center_freq = chandef->center_freq1;
+	u16 total = 2, center_freq = chandef->center_freq1;
 	u8 *cal = dev->cal, *eep = dev->mt76.eeprom.data;
+	int idx;
 
 	if (!(eep[MT_EE_DO_PRE_CAL] & MT_EE_WIFI_CAL_DPD))
 		return 0;