Fix error caused by non-existent variable_frequency property

Author: makermelissa

adafruit_motor/stepper.py

@@ -114,7 +114,7 @@ def __init__(
             self._coil = (ain2, bin1, ain1, bin2)
             for i in range(4):
                 if self._coil[i].frequency < 1500:
-                    if not self._coil[i].variable_frequency:
+                    if hasattr(self._coil[i], "variable_frequency") and not self._coil[i].variable_frequency:
                         raise ValueError(
                             "PWMOut outputs must either be set to at least "
                             "1500 Hz or allow variable frequency."