libata-core: fix kernel-doc warning

Fix libata-core kernel-doc warning:
Warning(linux-2.6.25-rc2-git6//drivers/ata/libata-core.c:168): No description found for parameter 'ap'

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