powerpc: Fix no interrupt handling in pata_of_platform

When no interrupt is specified the pata_of_platform fills the irq_res
resource with -1, which is wrong to do for two reasons:

1. By definition, 'no irq' should be IRQ 0, not some negative integer;
2. pata_platform checks for irq_res.start > 0, but since irq_res.start
   is unsigned type, the check will be true for `-1'.

Reported-by: Steven A. Falco <sfalco@harris.com>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/drivers/ata/pata_of_platform.c b/drivers/ata/pata_of_platform.c
index 408da30..1f18ad9 100644
--- a/drivers/ata/pata_of_platform.c
+++ b/drivers/ata/pata_of_platform.c
@@ -52,7 +52,7 @@
 
 	ret = of_irq_to_resource(dn, 0, &irq_res);
 	if (ret == NO_IRQ)
-		irq_res.start = irq_res.end = -1;
+		irq_res.start = irq_res.end = 0;
 	else
 		irq_res.flags = 0;