AHCI: ahci Driver crash in ahci_host_activate

[Suravee Suthikulanit <suravee.suthikulpanit@amd.com>]

I ran into a crash during booting with the latest 3.11.0-rc1.  After 
investigation, the crash was triggered by the patch below where the "pp" 
is NULL in some cases.

Suravee

commit b29900e62598cecd519c9ab2b8e4d03f8ebf702d
Author: Alexander Gordeev <agordeev@redhat.com>
Date:   Wed May 22 08:53:48 2013 +0900

     AHCI: Make distinct names for ports in /proc/interrupts

     Currently all interrupts assigned to AHCI ports show up in
     '/proc/interrupts' as 'ahci'. This fix adds port numbers as
     suffixes and hence makes the descriptions distinct.

     Reported-by: Jan Beulich <JBeulich@suse.com>
     Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
     Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
index 251e57d..4d1c672 100644
--- a/drivers/ata/ahci.c
+++ b/drivers/ata/ahci.c
@@ -1142,9 +1142,11 @@ int ahci_host_activate(struct ata_host *host, int 
irq, unsigned int n_msis)
                 return rc;

         for (i = 0; i < host->n_ports; i++) {
+               struct ahci_port_priv *pp = host->ports[i]->private_data;
+
                 rc = devm_request_threaded_irq(host->dev,
                         irq + i, ahci_hw_interrupt, ahci_thread_fn, 
IRQF_SHARED,
-                       dev_driver_string(host->dev), host->ports[i]);
+                       pp->irq_desc, host->ports[i]);
                 if (rc)
                         goto out_free_irqs;
         }