ata: libata-core: Detach a port devices on shutdown

Modify ata_pci_shutdown_one() to schedule EH to unload a port devices
before freezing and thawing the port. This ensures that drives are
cleanly disabled and transitioned to standby power mode when
a PCI adapter is removed or the system is powered off.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Tested-by: Chia-Lin Kao (AceLan) <acelan.kao@canonical.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>