x86/mce: Fix the MCE poll timer logic

In commit 82f7af09 (x86/mce: Cleanup timer mess), Thomas just forgot
the "/ 2" there while cleaning up.

Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>