x86, mce: don't init timer if !mce_available

In mce_cpu_restart, mce_init_timer is called unconditionally.
If !mce_available (e.g. mce is disabled), there are no useful work
for timer.  Stop running it.

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