Fix pid_type "Normal" crashing on startup (part 2)

Found while live-testing the previous fix: brewpi.py unconditionally
wired heater.power_set to tc.set_model_power, but "Normal" has no
internal model and thus no such method - same hasattr guard already
used elsewhere (tasks/sud.py's apply_plant_params) for this.

brewpi/brewpi.py

@@ -86,8 +86,9 @@ if __name__ == '__main__':
 	# Assign temp. sensor readings to tc
 	sensor_task.set_on_changed("temp", ChangedFloat(tc.set_theta_ist, prec=2).set)
 
-	# Assign heater power set to tc model
-	heater.set_on_changed("power_set", tc.set_model_power)
+	# Assign heater power set to tc model - "Normal" has no internal model
+	if hasattr(tc, "set_model_power"):
+		heater.set_on_changed("power_set", tc.set_model_power)
 
 	# For simulation
 	if "sim" in config['Controller']['sensor_name']: