Make reset methods and related functions deal with ata_link instead of
ata_port.

* ata_do_reset()
* ata_eh_reset()
* all prereset/reset/postreset methods and related functions

This patch introduces no behavior change.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>