PCI: aardvark: Fix return value of MSI domain .alloc() method

MSI domain callback .alloc() (implemented by advk_msi_irq_domain_alloc()
function) should return zero on success, since non-zero value indicates
failure.

When the driver was converted to generic MSI API in commit f21a8b1b
("PCI: aardvark: Move to MSI handling using generic MSI support"), it
was converted so that it returns hwirq number.

Fix this.

Link: https://lore.kernel.org/r/20211028185659.20329-3-kabel@kernel.org
Fixes: f21a8b1b ("PCI: aardvark: Move to MSI handling using generic MSI support")
Signed-off-by: NPali Rohár <pali@kernel.org>
Signed-off-by: NMarek Behún <kabel@kernel.org>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org

drivers/pci/controller/pci-aardvark.c

@@ -1138,7 +1138,7 @@ static int advk_msi_irq_domain_alloc(struct irq_domain *domain,
 				    domain->host_data, handle_simple_irq,
 				    NULL, NULL);
 
-	return hwirq;
+	return 0;
 }
 
 static void advk_msi_irq_domain_free(struct irq_domain *domain,