Enable use of FULL_ON and FULL_OFF bits.

It seems that the PCA9685 would not produce a 100% duty cycle. The highest achievale was 4095/4096 with a call to PCA9685::setPWM(?, 4095). PCA9685 will now use the FULL_ON and FULL_OFF bits in the PWM
registers when generating a 100% and 0% duty cycle respectivly.