ahci: disable correct irq for dummy ports

irq already contains the interrupt number for the port, don't add the
port index to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: d684a90d ("ahci: per-port msix support")
Cc: stable@vger.kernel.org v4.5+