ar71xx: fix ath79/rb4xx IRQ initialization on kernel 4.14

Apply the same approach as in commit 3b53d6fdbc ("ar71xx: fix pci irq
init on kernel 4.14") to fix IRQ initialization for ath79-based chipsets
on rb4xx.

Ref: PR#2182
Acked-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: W. Michael Petullo <mike@flyn.org>
[commit ref fix]
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 8c7d6c47a7)

target/linux/ar71xx/files/arch/mips/ath79/mach-rb4xx.c

@@ -88,7 +88,7 @@ static struct platform_device rb4xx_nand_device = {
 	.id	= -1,
 };
 
-static struct ath79_pci_irq rb4xx_pci_irqs[] __initdata = {
+static struct ath79_pci_irq rb4xx_pci_irqs[] = {
 	{
 		.slot	= 17,
 		.pin	= 1,