net: designware: Split the link init into a separate function

With rockchip we need to make adjustments after the link speed is set but
before enabling received/transmit. In preparation for this, split these
two pieces into separate functions.

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>