x86: hpet: Use irq_alloc/free_hwirq()

x86: hpet: Use irq_alloc/free_hwirq()

Use the new interfaces. No functional change.

This does not replace the requirement to move x86 to irq domains, but
it limits the mess to some degree.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Grant Likely <grant.likely@linaro.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org
Link: http://lkml.kernel.org/r/20140507154334.991589924@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>