driver core: add BUS_ATTR_WO() macro

Many bus attributes are write-only, so provide a simple macro for that
to be able to match the other driver core attribute macros.

Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>