x86: HPET: convert WARN_ON to WARN_ON_ONCE

It is possible to flood the console with call traces if the WARN_ON
condition is true because of the frequency with which this function is
called.

Signed-off-by: Matt Fleming <mjf@gentoo.org>
Cc: mingo@elte.hu
Cc: venkatesh.pallipadi@intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>