ata: libata-scsi: Cleanup ata_scsi_offline_dev()

Change the function ata_scsi_offline_dev() to return a bool and change
this function kdoc comment to have the correct mention of its call site.
No functional changes.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Niklas Cassel <cassel@kernel.org>