irqdomain: Check for NULL function pointer in irq_domain_free_irqs_hierarchy()

A follow-on patch will call irq_domain_free_irqs_hierarchy() when the
free() function pointer may be NULL.

Add a NULL pointer check to handle this new use case.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Link: http://lkml.kernel.org/r/1503017616-3252-5-git-send-email-david.daney@cavium.com