libata: add missing kernel-doc

Fix libata missing kernel-doc:

Warning(lin2628-rc2//drivers/ata/libata-core.c:4562): No description
found for parameter 'tag'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>