backlight: Fix backlight limiting on spitz/corgi devices

On spitz (& similar) machines, if battery is running low, backlight
needs
to be limited to lower step.  Unfortunately, current code uses &= for
limiting, turning backlight off completely for some backlight settings.
Fix that.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>