twister: Initialise unbound 'hardware' variable

hardware was referenced as return value, without being declared. When the
program goes into the 'except' path, hardware is unbound, but returned,
which throws an error:

`UnboundLocalError`: local variable 'hardware' referenced before assignment

Signed-off-by: Manoel Brunnen <manoel.brunnen@ithinx.io>

scripts/pylib/twister/twisterlib/handlers.py

@@ -452,6 +452,7 @@ class DeviceHandler(Handler):
                 logger.error("{} timed out".format(script))
 
     def get_hardware(self):
+        hardware = None
         try:
             hardware = self.device_is_available(self.instance)
             while not hardware: