irqchip/gic-v2m: Fix SPI release on error path

On failing to allocate the required SPIs, the actual number of interrupts
should be freed and not its log2 value.

Fixes: de337ee3 ("irqchip/gic-v2m: Add PCI Multi-MSI support")
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Yang Yingliang <yangyingliang@huawei.com>
Cc: Sumit Garg <sumit.garg@linaro.org>
Link: https://lkml.kernel.org/r/20180622095254.5906-4-marc.zyngier@arm.com