libata: fix libata-core.c kernel-doc warning

Fix kernel-doc warning in libata-core.c:

Warning(..//drivers/ata/libata-core.c:4763): No description found for parameter 'tag'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tejun Heo <tj@kernel.org>