x86: GART: pci-gart_64.c: Use correct length in strncmp

Signed-off-by: Joe Perches <joe@perches.com>
Cc: <stable@kernel.org> # .3x.x
LKML-Reference: <1257818330.12852.72.camel@Joe-Laptop.home>
Signed-off-by: Ingo Molnar <mingo@elte.hu>