vfio: platform: Switch to platform_get_irq_optional()

Since commit 7723f4c5 ("driver core: platform: Add an error
message to platform_get_irq*()"), platform_get_irq() calls dev_err()
on an error. As we enumerate all interrupts until platform_get_irq()
fails, we now systematically get a message such as:
"vfio-platform fff51000.ethernet: IRQ index 3 not found" which is
a false positive.

Let's use platform_get_irq_optional() instead.
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Cc: stable@vger.kernel.org # v5.3+
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/platform/vfio_platform.c

@@ -44,7 +44,7 @@ static int get_platform_irq(struct vfio_platform_device *vdev, int i)
 {
 	struct platform_device *pdev = (struct platform_device *) vdev->opaque;
 
-	return platform_get_irq(pdev, i);
+	return platform_get_irq_optional(pdev, i);
 }
 
 static int vfio_platform_probe(struct platform_device *pdev)