Fix enable logic to invert control state.

Author: tdicola

adafruit_si5351.py

@@ -426,7 +426,7 @@ def outputs_enabled(self):
 
     @outputs_enabled.setter
     def outputs_enabled(self, val):
-        if val:
+        if not val:
             self._write_u8(_SI5351_REGISTER_3_OUTPUT_ENABLE_CONTROL, 0xFF)
         else:
             self._write_u8(_SI5351_REGISTER_3_OUTPUT_ENABLE_CONTROL, 0x00)