Use timerqueue_iterate_next() to get to the next timer in
__hrtimer_next_event_base() without browsing the timerqueue
details diredctly.

No intentional changes in functionality.
Suggested-by: NFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>