ata: libata-core: Do not try to set sleeping devices to standby

In ata ata_dev_power_set_standby(), check that the target device is not
sleeping. If it is, there is no need to do anything.

Fixes: aa3998db

 ("ata: libata-scsi: Disable scsi device manage_system_start_stop")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>