cpu/atmega_common/periph/pwm: Minor fix & cleanup
cpu/atmega_common/periph/pwm: Minor fix & cleanup

- On pwm_poweron, the PWM resolution was not restored. (A custom resolution was
  only usable if, PWM channel 0 is not used. That configuration is not common,
  so this bug was likely never triggered)
- Disabled a work around to prevent flickering:
    - Previously, PWM was disconnected on level 0% and 100%
    - This increases the run time of `pwm_set()`
    - It prevents using the PWM for wave form generation via DDS, as the wave
      noticeably jumps when reaching 0% or 100%
- Slightly reduces memory requirements: 2 Bytes of RAM, 112 Bytes of ROM
    - Tested with avr-gcc 9.2.0 and LTO enabled

jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,kYc0o/RIOT,basilfx/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kYc0o/RIOT,basilfx/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,basilfx/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,jasonatran/RIOT,miri64/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,basilfx/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kYc0o/RIOT,miri64/RIOT,authmillenon/RIOT,kYc0o/RIOT,OTAkeys/RIOT