x86: fix section mismatch warning in mcheck/mce_amd_64.c

Fix following warning:
WARNING: arch/x86/kernel/cpu/mcheck/built-in.o(.text+0x1584): Section mismatch: reference to .cpuinit.text:threshold_create_device in 'threshold_cpu_callback'

threshold_cpu_callback() is only used by threshold_cpu_notifier.
threshold_cpu_notifier is only used for cpu hot plug as it is registered
using register_hotcpu_notifier().

Mark them both __cpuinit to fix the warning.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>