aty128fb: test below 0 on unsigned pll->post_divider

pll->post_divider is unsigned, so the test fails

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Antonino Daplas <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>