libata-pmp: update ata_eh_reset() for PMP

PMP always requires SRST to be enabled.  Also, hardreset reports
classification code from the first device when PMP is attached, not
from the PMP.  Update ata_eh_reset() such that followup softreset is
performed if the controller is PMP capable and the host link is being
reset.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>