Fix a erroneous calculation of the legacy brightness values as reported by
Paul Collins.  Additionally, it moves the calculation of the negative value
in the radeonfb driver after the value check.

Signed-off-by: Michael Hanselmann <linux-kernel@hansmi.ch>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Paul Collins <paul@briny.ondioline.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>