libata: forbid port runtime pm by default, fixing regression

commit 0c8d32c2 upstream.

Forbid port runtime pm by default because it has known hotplug issue.
User can allow it by, for example

echo auto > /sys/devices/pci0000:00/0000:00:1f.2/ata2/power/control

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>