carlo_gavazzi: set minimum timeout to 500 ms

The network stack in the meter can lock up if too small a timeout
is used.  Make sure this remains unlikely.

carlo_gavazzi.py

@@ -41,6 +41,7 @@ def decode(self, values):
 class EM24_Meter(device.EnergyMeter):
     productid = 0xb017
     productname = 'Carlo Gavazzi EM24 Ethernet Energy Meter'
+    min_timeout = 0.5
 
     def __init__(self, *args):
         super(EM24_Meter, self).__init__(*args)