comedi: drivers: ni_routes: Demote non-conforming kernel-doc headers

comedi: drivers: ni_routes: Demote non-conforming kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/staging/comedi/drivers/ni_routes.c:249: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 drivers/staging/comedi/drivers/ni_routes.c:398: warning: Function parameter or member 'routes' not described in 'ni_route_set_has_source'
 drivers/staging/comedi/drivers/ni_routes.c:398: warning: Function parameter or member 'source' not described in 'ni_route_set_has_source'
 drivers/staging/comedi/drivers/ni_routes.c:524: warning: Function parameter or member 'src_sel_reg_value' not described in 'ni_find_route_source'
 drivers/staging/comedi/drivers/ni_routes.c:524: warning: Function parameter or member 'dest' not described in 'ni_find_route_source'
 drivers/staging/comedi/drivers/ni_routes.c:524: warning: Function parameter or member 'tables' not described in 'ni_find_route_source'

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Alexander A. Klimov" <grandmaster@al2klimov.de>
Cc: "Spencer E. Olson" <olsonse@umich.edu>
Cc: linux-staging@lists.linux.dev
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210520122538.3470259-5-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>