i386: PIT stop only, when in periodic or oneshot mode

The patch is necessary on one of my boxen, where programming the stop
sequence twice leads to PIT malfunction.

Sigh !

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andi Kleen <ak@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>