irqchip/loongson-pch-msi: Remove unneeded variable

irq_domain_alloc_irqs_parent() returns 0 on success and non-zero value
on failure, it is redudant to check its non-zero return value and then
return it, so just remove the variable "ret" and return directly in the
function pch_msi_parent_domain_alloc().

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1594087972-21715-7-git-send-email-yangtiezhu@loongson.cn