[PATCH] libata: make ata_dev_knobble() per-device

ata_dev_knobble() unconditionally used the first device of the port to
determine whether a device is bridged or not.  This causes bridge
limit to be incorrectly applied or unapplied for hosts with slave
devices (e.g. ata_piix).

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>