Fix set_pwm_freq() for ATtinys

[caternuson]

For #116 (got issue number wrong in PR fork name)

NOTE: The pwm_pins for attiny8x7 and attinyx16 have been set to include pins for both 8 bit PWM mode and 16 bit PWM mode (a more recent firmware feature). However, only one mode will actually be built into the firmware. There is currently no way to query this from the firmware so that pwm_pins could be adjusted at runtime.

Tested on an ATtiny816 seesaw breakout with this code:

import board
from adafruit_seesaw import seesaw
ss = seesaw.Seesaw(board.STEMMA_I2C())
ss.set_pwm_freq(11, 50)

BEFORE
no output, so waiting forever to trigger

AFTER
expected output on pin 11

[tannewt]

Thank you!