irqchip/armada-370-xp: Fix return value of armada_370_xp_msi_alloc()

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

Signed-off-by: Pali Rohár <[e-mail]>
Fixes: fcc392d501bd ("irqchip/armada-370-xp: Use the generic MSI infrastructure")
Cc: [e-mail]
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/irqchip/irq-armada-370-xp.c

···
 				    NULL, NULL);
 	}
 
-	return hwirq;
+	return 0;
 }
 
 static void armada_370_xp_msi_free(struct irq_domain *domain,