Sets all PWM channels.

def set_all_pwm(self, on, off):
        """Sets all PWM channels."""
        self.i2c.write8(ALL_LED_ON_L, on & 0xFF)
        self.i2c.write8(ALL_LED_ON_H, on >> 8)
        self.i2c.write8(ALL_LED_OFF_L, off & 0xFF)
        self.i2c.write8(ALL_LED_OFF_H, off >> 8)