moved default initalization of _devices to clean init code

hpib.py

@@ -18,6 +18,7 @@ class PortDevice(object):
 class PrologixGpibUsb(PortDevice):
     _serialPort = None
     _hpib_address = -1
+    _devices = "Not scanned."
 
     def __init__(self, port="", timeout=1, port_format="%d", max_device_num=9, verbose=False,
                  find_devices=False):
@@ -36,8 +37,6 @@ class PrologixGpibUsb(PortDevice):
 
         if find_devices:
             self._find_devices()
-        else:
-            self._devices = "Not scanned."
 
     def _set_default_platform_values(self):
         if platform.system() == "Linux":