bluetooth: Return int8_t and use -128 for unknown tx power.

device/bluetooth/bluetooth_classic_device_mac.h

Index: device/bluetooth/bluetooth_classic_device_mac.h
diff --git a/device/bluetooth/bluetooth_classic_device_mac.h b/device/bluetooth/bluetooth_classic_device_mac.h
index acd1dca315e7a0a2d6128b5129293fd986a6e905..06aaec936739affad337cd9cef3c482aa602a31b 100644
--- a/device/bluetooth/bluetooth_classic_device_mac.h
+++ b/device/bluetooth/bluetooth_classic_device_mac.h
@@ -41,8 +41,8 @@ class BluetoothClassicDeviceMac : public BluetoothDeviceMac {
   bool IsConnectable() const override;
   bool IsConnecting() const override;
   UUIDList GetUUIDs() const override;
-  int16_t GetInquiryRSSI() const override;
-  int16_t GetInquiryTxPower() const override;
+  int8_t GetInquiryRSSI() const override;
+  int8_t GetInquiryTxPower() const override;
   bool ExpectingPinCode() const override;
   bool ExpectingPasskey() const override;
   bool ExpectingConfirmation() const override;