ahci: restore pci_intx() handling

Commit a5bfc471

 dropped explicit
pci_intx() manipulation from ahci because it seemed unnecessary and
ahci doesn't seem to be the right place to be tweaking it if it were.
This was largely okay but there are exceptions.  There was one on an
embedded platform which was fixed via firmware and now bko#14124
reports it on a HP DL320.

  http://bugzilla.kernel.org/show_bug.cgi?id=14124

I still think this isn't something libata drivers should be caring
about (the only ones which are calling pci_intx() explicitly are
libata ones and one other driver) but for now reverting the change
seems to be the right thing to do.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>