Fix Smith Predictor Controller Bug

Fix errant variable naming.

Author: nebhead

controller/pid_sp.py

@@ -79,7 +79,7 @@ def __init__(self, config, units, cycle_data):
 		self.set_target(0.0)
 
 	def _calculate_gains(self, pb, ti, td):
-		if pd == 0:
+		if pb == 0:
 			self.kp = 0
 		else:
 			self.kp = -1 / pb