libata: ata_dev_disable() should be called from EH context

ata_port_detach() calls ata_dev_disable() with host lock held but
ata_dev_disable() should be called from EH context.  ata_port_detach()
steals EH context by setting ATA_PFLAG_UNLOADAING and flushing EH.
Drop locking around ata_dev_disable() and note that ata_port_detach()
owns EH context at that point.

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