libata: fix kernel-doc parameter warning

Fix libata kernel-doc parameter:

Warning(linux-2.6.25-rc2-git3//drivers/ata/libata-scsi.c:845): No description found for parameter 'rq'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>