x86/cpu: Fix overrun check in arch_print_cpu_modalias()

snprintf() does not return a negative value when truncating.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Thomas Renninger <trenn@suse.de>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>