ahci: fix Null pointer dereference in achi_host_active()

commit b29900e6

 (AHCI: Make distinct names for ports in /proc/interrupts)
introuded a regression, which resulted Null pointer dereference for achi
host with dummy ports. For ahci ports, when the port is dummy port, its
private_data will be NULL, as ata_dummy_port_ops doesn't support ->port_start.

changes in v2: use pp to check dummy ports, update comments

Reported-and-tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Xiaotian Feng <xtfeng@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Alexander Gordeev <agordeev@redhat.com>
Cc: linux-ide@vger.kernel.org
Cc: linux-kernel@vger.kernel.org