[PATCH] libata: add dev->ap

Add dev->ap which points back to the port the device belongs to.  This
makes it unnecessary to pass @ap for silly reasons (e.g. printks).
Also, this change is necessary to accomodate later PM support which
will introduce ATA link inbetween port and device.

Signed-off-by: Tejun Heo <htejun@gmail.com>