of: make of_device_make_bus_id() static

of_device_make_bus_id() was changed to non-static by commit c6601225


("of/device: Make of_device_make_bus_id() usable by other code") more than
6 years ago, but there are no users of it outside of platform.c.  Make the
function static again.

Signed-off-by: Frank Rowand <frank.rowand@am.sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>