genirq: do not execute DEBUG_SHIRQ when irq setup failed

When requesting an IRQ, the DEBUG_SHIRQ code executes a fake IRQ just to make
sure the driver is ready to receive an IRQ immediately.  The problem was that
this fake IRQ was being executed even if interrupt line failed to be allocated
by __setup_irq.

Signed-off-by: Luis Henriques <henrix@sapo.pt>
LKML-Reference: <20090401170635.GA4392@hades.domain.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[ fixed bug pointed out by a warning reported by Stephen Rothwell ]
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>