fbdev/atyfb: fix recent breakage in correct_chipset()

The 6e36308a

 "fb: fix atyfb build warning" isn't right.  It makes all
the indexes off by one.  This patch reverts it and casts the
ARRAY_SIZE() to int to silence the build warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>