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

drivers/ata/libata-core.c

@@ -4557,6 +4557,7 @@ void swap_buf_le16(u16 *buf, unsigned int buf_words)
 /**
  *	ata_qc_new_init - Request an available ATA command, and initialize it
  *	@dev: Device from whom we request an available command structure
+ *	@tag: command tag
  *
  *	LOCKING:
  *	None.