libata-link: update hotplug to handle PMP links

Update hotplug to handle PMP links.  When PMP is attached, the PMP
number corresponds to C of SCSI H:C:I:L.  While at it, change argument
to ata_find_dev() to @devno from @id to avoid confusion with SCSI
device ID.

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