libata: initialize port_task when !CONFIG_ATA_SFF

ap->port_task was not initialized if !CONFIG_ATA_SFF later triggering
lockdep warning.  Make sure it's initialized.

Reported by Larry Finger.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>