powerpc/eeh: Fix crashes in eeh_report_resume()

The notify_resume() callback in eeh_ops is NULL on powernv, leading to
crashes:

  NIP (null)
  LR  eeh_report_resume+0x218/0x220
  Call Trace:
   eeh_report_resume+0x1f0/0x220 (unreliable)
   eeh_pe_dev_traverse+0x98/0x170
   eeh_handle_normal_event+0x3f4/0x650
   eeh_handle_event+0x54/0x380
   eeh_event_handler+0x14c/0x210
   kthread+0x168/0x1b0
   ret_from_kernel_thread+0x5c/0xb4

Fix it by adding a check before calling it.

Fixes: 856e1eb9 ("PCI/AER: Add uevents in AER and EEH error/resume")
Signed-off-by: Juan J. Alvarez <jjalvare@linux.vnet.ibm.com>
Reviewed-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Tested-by: Carol L. Soto <clsoto@us.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Tested-by: Mauro S. M. Rodrigues <maurosr@linux.vnet.ibm.com>
Acked-by: Michael Neuling <mikey@neuling.org>
[mpe: Rewrite change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/eeh_driver.c

@@ -384,7 +384,8 @@ static void *eeh_report_resume(void *data, void *userdata)
 	eeh_pcid_put(dev);
 	pci_uevent_ers(dev, PCI_ERS_RESULT_RECOVERED);
 #ifdef CONFIG_PCI_IOV
-	eeh_ops->notify_resume(eeh_dev_to_pdn(edev));
+	if (eeh_ops->notify_resume && eeh_dev_to_pdn(edev))
+		eeh_ops->notify_resume(eeh_dev_to_pdn(edev));
 #endif
 	return NULL;
 }