fix freq

Author: brentru

src/adafruit_blinka/microcontroller/generic_agnostic_board/PWMOut.py

@@ -29,6 +29,7 @@ def _open(self, pin, duty=0, freq=500, variable_frequency=False):
 
         # set frequency
         self.frequency = freq
+        self._variable_frequency = variable_frequency
         # set duty
         self.duty_cycle = duty