ahci: port_no should be used when clearing IRQ in ahci_thaw()

ap->id is logcial port ID which is unique among all ATA ports and
doesn't have anything to do with hardware port index.  ap->port_no is
the hardware port index and thus should be used when clearing IRQ mask
in ahci_thaw().

This problem has been spotted by Jeff Garzik <jgarzik@pobox.com>.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>