powerpc/eeh: Fix unbalanced enable for IRQ

The patch fixes following issue:

Unbalanced enable for IRQ 23
------------[ cut here ]------------
WARNING: at kernel/irq/manage.c:437
:
NIP [c00000000016de8c] .__enable_irq+0x11c/0x140
LR [c00000000016de88] .__enable_irq+0x118/0x140
Call Trace:
[c000003ea1f23880] [c00000000016de88] .__enable_irq+0x118/0x140 (unreliable)
[c000003ea1f23910] [c00000000016df08] .enable_irq+0x58/0xa0
[c000003ea1f239a0] [c0000000000388b4] .eeh_enable_irq+0xc4/0xe0
[c000003ea1f23a30] [c000000000038a28] .eeh_report_reset+0x78/0x130
[c000003ea1f23ac0] [c000000000037508] .eeh_pe_dev_traverse+0x98/0x170
[c000003ea1f23b60] [c0000000000391ac] .eeh_handle_normal_event+0x2fc/0x3d0
[c000003ea1f23bf0] [c000000000039538] .eeh_handle_event+0x2b8/0x2c0
[c000003ea1f23c90] [c000000000039600] .eeh_event_handler+0xc0/0x170
[c000003ea1f23d30] [c0000000000da9a0] .kthread+0xf0/0x100
[c000003ea1f23e30] [c00000000000a1dc] .ret_from_kernel_thread+0x5c/0x80
Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/eeh_driver.c

@@ -143,10 +143,14 @@ static void eeh_disable_irq(struct pci_dev *dev)
 static void eeh_enable_irq(struct pci_dev *dev)
 {
 	struct eeh_dev *edev = pci_dev_to_eeh_dev(dev);
+	struct irq_desc *desc;
 
 	if ((edev->mode) & EEH_DEV_IRQ_DISABLED) {
 		edev->mode &= ~EEH_DEV_IRQ_DISABLED;
-		enable_irq(dev->irq);
+
+		desc = irq_to_desc(dev->irq);
+		if (desc && desc->depth > 0)
+			enable_irq(dev->irq);
 	}
 }