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

device/bluetooth/bluetooth_low_energy_device_mac.h

Index: device/bluetooth/bluetooth_low_energy_device_mac.h
diff --git a/device/bluetooth/bluetooth_low_energy_device_mac.h b/device/bluetooth/bluetooth_low_energy_device_mac.h
index 4466d67f13f9181d2adb21cd08994fe6d6f90546..3af5ed7fc148511c9ec4aece62e608c3d987d2a0 100644
--- a/device/bluetooth/bluetooth_low_energy_device_mac.h
+++ b/device/bluetooth/bluetooth_low_energy_device_mac.h
@@ -53,8 +53,8 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothLowEnergyDeviceMac
   bool IsConnectable() const override;
   bool IsConnecting() const override;
   BluetoothDevice::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;