ata: libata-core: Do not call ata_dev_power_set_standby() twice

For regular system shutdown, ata_dev_power_set_standby() will be
executed twice: once the scsi device is removed and another when
ata_pci_shutdown_one() executes and EH completes unloading the devices.

Make the second call to ata_dev_power_set_standby() do nothing by using
ata_dev_power_is_active() and return if the device is already in
standby.

Fixes: 2da4c5e2

 ("ata: libata-core: Improve ata_dev_power_set_active()")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>