x86, mce: Rename incorrect macro name "CONFIG_X86_THRESHOLD"

CONFIG_X86_THRESHOLD used in arch/x86/kernel/irqinit.c is always
undefined.  Rename it to the correct name "CONFIG_X86_MCE_THRESHOLD".

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Reviewed-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Andi Kleen <andi@firstfloor.org>
LKML-Reference: <4A667FD4.3010509@hitachi.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>