QBluetoothSocket/bluez: fix QString::arg() usage

QString::arg() returns the new value but leaves *this unchanged.

Found by GCC 7 (-Wunused-result).

Change-Id: I8a0ddb801ded94365f0e9a7ae13a37a96c7f437a
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/bluetooth/qbluetoothsocket_bluez.cpp

@@ -251,8 +251,7 @@ void QBluetoothSocketPrivate::_q_writeNotify()
                 break;
             default:
                 // every other case returns error
-                errorString = QBluetoothSocket::tr("Network Error: %1");
-                errorString.arg(qt_error_string(errno));
+                errorString = QBluetoothSocket::tr("Network Error: %1").arg(qt_error_string(errno)) ;
                 q->setSocketError(QBluetoothSocket::NetworkError);
                 break;
             }
@@ -507,8 +506,7 @@ qint64 QBluetoothSocketPrivate::writeData(const char *data, qint64 maxSize)
                 sz = 0;
                 break;
             default:
-                errorString = QBluetoothSocket::tr("Network Error: %1");
-                errorString.arg(qt_error_string(errno));
+                errorString = QBluetoothSocket::tr("Network Error: %1").arg(qt_error_string(errno));
                 q->setSocketError(QBluetoothSocket::NetworkError);
             }
         }