[POWERPC] Fix for via-pmu based backlight control

This fixes a few issues with via-pmu based backlight control.

First, it fixes a sign problem with the setup of the backlight
curve since the `range' value there -can- (and will) go negative.

Then, it reworks the interaction between this and the via-pmu sleep
code to properly restore backlight on wakeup from sleep.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>