Correct the use of QString::arg

It returns an updated QString, without modifying the variable it was
called on. Found by Clang when building:

qbluetoothsocket_bluez.cpp:255:17: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]

Change-Id: I27b55fdf514247549455fffd14b1795de236d488
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>