x86/hpet: Migrate to new set_state interface

x86/hpet: Migrate to new set_state interface

Migrate hpet driver to the new 'set-state' interface provided by
clockevents core, the earlier 'set-mode' interface is marked obsolete
now.

This also enables us to implement callbacks for new states of clockevent
devices, for example: ONESHOT_STOPPED.

Forward definition of 'hpet_clockevent' wasn't required and so it is
placed after all the callback are defined, to avoid forward declaring
all the callbacks.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linaro-kernel@lists.linaro.org
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: http://lkml.kernel.org/r/8cc9864b6d6342dfac28f270cf69f4cba46fffae.1437042675.git.viresh.kumar@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>