ahci: Add check for device presence (PCIe hot unplug) in ahci_stop_engine()

Exit directly with ENODEV, if the AHCI controller is not available
anymore. Otherwise a delay of 500ms for each port is added to the remove
function while trying to issue a command on the non-existent controller.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>