x86, mce: cleanup mce_start()

Simplify interface of mce_start():

-       no_way_out = mce_start(no_way_out, &order);
+       order = mce_start(&no_way_out);

Now Monarch and Subjects share same exit(return) in usual path.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>