rapidio: rio-sysfs.c: convert to use BUS_ATTR_WO

We are trying to get rid of BUS_ATTR() and the usage of that in
rio-sysfs.c can be trivially converted to use BUS_ATTR_WO(), so use that
instead.

Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Alexandre Bounine <alex.bou9@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>