iommu/rockchip: Don't provoke WARN for harmless IRQs

Although we don't generally expect IRQs to fire for a suspended IOMMU,
there are certain situations (particularly with debug options) where
we might legitimately end up with the pm_runtime_get_if_in_use() call
from rk_iommu_irq() returning 0. Since this doesn't represent an actual
error, follow the other parts of the driver and save the WARN_ON()
condition for a genuine negative value. Even if we do have spurious
IRQs due to a wedged VOP asserting the shared line, it's not this
driver's job to try to second-guess the IRQ core to warn about that.

Reported-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>