PCI: iproc: Fix return value of iproc_msi_irq_domain_alloc()

PCI: iproc: Fix return value of iproc_msi_irq_domain_alloc()

IRQ domain alloc function should return zero on success. Non-zero value
indicates failure.

Link: https://lore.kernel.org/r/20210303142202.25780-1-pali@kernel.org
Fixes: 578bd7cff172 ("PCI: iproc: Allow allocation of multiple MSIs")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Krzysztof Wilczyński <kw@linux.com>
Acked-by: Ray Jui <ray.jui@broadcom.com>
Acked-by: Marc Zyngier <maz@kernel.org>