Move the ATA link transport device related functions after the ATA
transport device related functions to avoid the need for forward
declaring ata_tdev_add() and ata_tdev_delete().

And while at it, do the following:
1) Change ata_is_ata_dev() and ata_is_link() to return a boolean
2) Fix a pointer declaration style in ata_is_ata_dev()
3) Improve the kdoc comments for ata_tdev_free(), ata_tdev_delete(),
   ata_tdev_add(), ata_tlink_delete() and ata_tlink_add()

No functional changes are introduced by this cleanup.
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Niklas Cassel <cassel@kernel.org>