irqchip/ls-scfg-msi: Fix MSI affinity handling

The ls-scfs-msi driver is not dealing with the effective affinity
as it should. Let's fix that, and make it clear that the effective
affinity is restricted to a single CPU. Also prevent the driver from
messing with the internals of the affinity setting infrastructure.

Reported-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Yang Yingliang <yangyingliang@huawei.com>
Cc: Sumit Garg <sumit.garg@linaro.org>
Link: https://lkml.kernel.org/r/20180622095254.5906-3-marc.zyngier@arm.com