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

device/bluetooth/dbus/bluetooth_device_client.cc

Index: device/bluetooth/dbus/bluetooth_device_client.cc
diff --git a/device/bluetooth/dbus/bluetooth_device_client.cc b/device/bluetooth/dbus/bluetooth_device_client.cc
index 11626f505a7be6f202afbdd338256831215fccb7..6663f9a3fccc5234b316ecda0c28003b2f639834 100644
--- a/device/bluetooth/dbus/bluetooth_device_client.cc
+++ b/device/bluetooth/dbus/bluetooth_device_client.cc
@@ -18,8 +18,10 @@ namespace bluez {
 
 namespace {
 
-// Value returned for the the RSSI or TX power if it cannot be read.
-const int kUnknownPower = 127;
+// The value returned if the RSSI cannot be read.
+constexpr int8_t kUnknownRSSI = 127;
+// The value returned if the TxPower cannot be read.
+constexpr int8_t kUnknownTxPower = -128;
 
 }  // namespace
 
@@ -320,9 +322,9 @@ class BluetoothDeviceClientImpl : public BluetoothDeviceClient,
   // Called when a response for the GetConnInfo method is received.
   void OnGetConnInfoSuccess(const ConnInfoCallback& callback,
                             dbus::Response* response) {
-    int16_t rssi = kUnknownPower;
-    int16_t transmit_power = kUnknownPower;
-    int16_t max_transmit_power = kUnknownPower;
+    int16_t rssi = kUnknownRSSI;
+    int16_t transmit_power = kUnknownTxPower;
+    int16_t max_transmit_power = kUnknownTxPower;
 
     if (!response) {
       LOG(ERROR) << "GetConnInfo succeeded, but no response received.";