rp2/machine_pwm: Fix PWM frequency setting.
rp2/machine_pwm: Fix PWM frequency setting.

The top value was off by 1: in order to count n ticks it has to be set to
n-1.

Fixes issue #8122.

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython