ata: core: fix irq description on AHCI single irq systems

On my machine with single irq AHCI just the PCI id is printed as
description in /proc/interrupts.
I found a related discussion from beginning of this year:
http://www.gossamer-threads.com/lists/linux/kernel/2117335



Seems like 4f37b504 ("libata: Use dev_name() for request_irq() to
distinguish devices") tried to fix displaying a proper interrupt
description for one scenario but broke it for another one.

The mentioned discussion ended in the current situation being
considered as broken but w/o a patch to fix it.

The following patch is based on a proposal in this mail thread.
Now the interrupt is properly described as:
PCI-MSI 512000-edge      ahci[0000:00:1f.2]

By combining both values also the scenario that commit 4f37b504
("libata: Use dev_name() for request_irq() to distinguish devices")
refers to should still be fine. There it should look like this now:
ahci[20100000.ide]

Using managed memory allocation ensures that the irq description
lives at least as long as the interrupt.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>