irqchip/bcm2836: Tolerate IRQs while no flag is set in ISR

On my RPi2 I got a lot of:
unexpected IRQ trap at vector 00

This happens because bcm2836_arm_irqchip_handle_irq() is sometimes
invoked even if the ISR is clear, and this case is not handled.

This patch explicitly handle this case, fixing the kernel complaints
about the bad IRQ lookup.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Lee Jones <lee@kernel.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-rpi-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1451166444-11044-4-git-send-email-eric@anholt.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>