dm: net: Tidy up designware driver ready for driver model

Adjust the error handling to use errno.h instead of returning -1. Change
leaf functions to pass in the arguments they require rather than struct
eth_device. Apart from simplifying the code it makes is easier to reuse
these functions for driver model, since mostly they actually only use
struct dw_eth_priv (which we can keep).

Create a stub for each Ethernet operation function. This will allow use to
share code with the driver model versions.

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