Fix logic for checking PWMOut

Thanks @jepler for catching the faulty logic!

Author: tekktrik

adafruit_motor/stepper.py

@@ -113,13 +113,12 @@ def __init__(
             # set a safe pwm freq for each output
             self._coil = (ain2, bin1, ain1, bin2)
             for i in range(4):
-                if self._coil[i].frequency < 1500 and self._coil[i].variable_frequency:
-                    self._coil[i].frequency = 2000
-                else:
+                if self._coil[i].frequency < 1500 and not self._coil[i].variable_frequency:
                     raise ValueError(
                         "PWMOut outputs must either be set to at least "
                         "1500 Hz or allow variable frequency."
                     )
+                self._coil[i].frequency = 2000
             if microsteps < 2:
                 raise ValueError("Microsteps must be at least 2")
             if microsteps % 2 == 1: