rapidio: use is_visible() to hide switch-specific attributes

Instead of creating switch-specific attributes by hand, implement
is_visible() method of attribute group and hide them when dealing with
non-switch devices. This will ensure that all attributes are created
together, before userspace gets notified of new device.

Also, remove rio-sysfs.c from list of files that are scanned when compiling
RapiodIO documentations as it no longer has any structured comments, and
leaving it in leads to warning when building docs.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>