genirq: add unlocked version of set_irq_handler()

Add unlocked version for use by irq_chip.set_type handlers which may
wish to change handler to level or edge handler when IRQ type is
changed.

The normal set_irq_handler() call cannot be used because it tries to
take irq_desc.lock which is already held when the irq_chip.set_type
hook is called.

Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>