x86: hpet: Make WARN_ON understandable

Andrew complained rightly that the WARN_ON in hpet_next_event() is
confusing and the code comment not really helpful.

Change it to WARN_ONCE and print the reason in clear text. Change the
comment to explain what kind of hardware wreckage we deal with.

Pointed-out-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Venki Pallipadi <venkatesh.pallipadi@intel.com>