hwmon: attribute is not polled after failure

This line of code was removed with revert bd01c24d (original efc68970)
but should not have been.  The line was originally added with 6956bbd4.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/pyhwmon/hwmon.py b/pyhwmon/hwmon.py
index e5dc19a..bb363dc 100644
--- a/pyhwmon/hwmon.py
+++ b/pyhwmon/hwmon.py
@@ -65,6 +65,7 @@
 				self.writeAttribute(attribute,rtn[1])
 		except:
 			print "HWMON: Attibute no longer exists: "+attribute
+			self.sensors.pop(objpath,None)
 			return False