Fix for screen backlight not working on Planet Computers Astro Slide PDA

This fixes the backlight working on these PDA computers

https://store.planetcom.co.uk/products/astro-slide

Taken from Gemian project "Debian on the Gemini PDA and Cosmo Communicator" at the request of the author

https://github.com/gemian/powerdevil/commit/8d7a18c7c31a5eea9e9ff1b90394f453d755a110