x86, mce: trivial clean up for mce_amd_64.c

Fix for followings:

WARNING: Use #include <linux/percpu.h> instead of <asm/percpu.h>
+#include <asm/percpu.h>

ERROR: Macros with multiple statements should be enclosed in a do - while
loop
+#define THRESHOLD_ATTR(_name, _mode, _show, _store)                    \
+{                                                                      \
+       .attr   = {.name = __stringify(_name), .mode = _mode },         \
+       .show   = _show,                                                \
+       .store  = _store,                                               \
+};

WARNING: usage of NR_CPUS is often wrong - consider using cpu_possible(),
num_possible_cpus(), for_each_possible_cpu(), etc
+       if (cpu >= NR_CPUS)

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>