Update setPWMFreq for ATTiny variants

[caternuson]

For #98.

Fixes logic in setPWMFreq() to support the ATTiny variants, similar to same logic found in analogWrite().

Tested with following example sketch running on a QT PY M0 attached to an ATTiny1616 dev breakout (PID 5690) and scoping pin 1:

#include <Adafruit_seesaw.h>

Adafruit_seesaw ss;

void setup() {
  Serial.begin(115200);
  while (!Serial);
  
  Serial.println("Seesaw PWM Test.");

  if (!ss.begin()) {
    Serial.println("Failed to init seesaw.");
    while(1);
  }
  Serial.println("Seesaw initd.");
}

void loop() {
  ss.setPWMFreq(1, 100);
  delay(2000);
  ss.setPWMFreq(1, 300);
  delay(2000);
}

BEFORE
no output

AFTER
expected output

[caternuson]

@ladyada for review