atyfb: Fix XCLK frequency on Apple iBook1

Fix a regression on Apple iBook1.  Changes in the clock init code caused an
incorrect XCLK frequency to be used leading to a corrupted display.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Cc: Olaf Hering <olaf@aepfle.de>
Cc: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>