spitz backlight: fix brightness limiting

[Richard Purdie]

On Thu, 2009-10-22 at 04:20 +0800, Eric Miao wrote:
> It's a little bit weird it's called 'limit_mask' when I first converted it
> to a spi driver. There must be some reasons, Richard, you've got
> any ideas?

Its a historical artifact that once worked through some bitfield magic
but probably stopped working when we increased the range of values the
backlight could accept and we didn't notice the limiting problem.

I'll happily take a patch to fix that.

Cheers,

Richard