net: designware: Export the operation functions

Export all functions so that drivers can use them, or not, as the need
arises.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Romain Perier <romain.perier@collabora.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>