mt76: mt7615: Fix assigning negative values to unsigned variable

Smatch reports the following:
drivers/net/wireless/mediatek/mt76/mt7615/mac.c:1865
mt7615_mac_adjust_sensitivity() warn: assigning (-110) to unsigned
variable 'def_th'
drivers/net/wireless/mediatek/mt76/mt7615/mac.c:1865
mt7615_mac_adjust_sensitivity() warn: assigning (-98) to unsigned
variable 'def_th'
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7615/mac.c

@@ -1869,7 +1869,7 @@ mt7615_mac_adjust_sensitivity(struct mt7615_phy *phy,
 	struct mt7615_dev *dev = phy->dev;
 	int false_cca = ofdm ? phy->false_cca_ofdm : phy->false_cca_cck;
 	bool ext_phy = phy != &dev->phy;
-	u16 def_th = ofdm ? -98 : -110;
+	s16 def_th = ofdm ? -98 : -110;
 	bool update = false;
 	s8 *sensitivity;
 	int signal;