Fix setting brightness to max value and brightness fading loop condition
Fix setting brightness to max value and brightness fading loop condition

- The `_denormalize_brightness` function does not clamp the output to
  the normal 0-255 value range. Force-clamped 0-255 now.
- The `_normalize_brightness` function does not clamp the output to the
  normal 0-100 percent range. Force-clamped 0-100 now.
- In the `@brightness.setter`, the while loop looks only for
  `current_value != value`. In some circumstances, it won't stop
  incrementing. Hard-clamped to 0-100 percent range.

Fixes #34.
linusg/rpi-backlight