Frequency of PCA9685 can't be set on a per channel basis

[kattni]

Introduced in #7

Expecting a setter, but there is no setter on line 77/78.

Adafruit_CircuitPython_Motor/adafruit_motor/stepper.py

Line 77 in e0b709f

for i in range(4):

We think it should check, and if it's wrong and it can't change it, throw an exception.