rtc: Fix hrtimer deadlock

Ben reported a lockup related to rtc. The lockup happens due to:

CPU0                                        CPU1

rtc_irq_set_state()			    __run_hrtimer()
  spin_lock_irqsave(&rtc->irq_task_lock)    rtc_handle_legacy_irq();
					      spin_lock(&rtc->irq_task_lock);
  hrtimer_cancel()
    while (callback_running);

So the running callback never finishes as it's blocked on
rtc->irq_task_lock.

Use hrtimer_try_to_cancel() instead and drop rtc->irq_task_lock while
waiting for the callback. Fix this for both rtc_irq_set_state() and
rtc_irq_set_freq().

Cc: stable@kernel.org
Reported-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>