atyfb: correct_chipset() can fail

Atari probe code relies on correct_chipset() failing if the device is not
a mach64 GX/CX.  aty_chips[] array would be indexed with -1 in that case.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>