net: e1000: Prepare for driver model conversion

Since struct eth_device does not exist with CONFIG_DM_ETH defined, avoid
using it in the driver unless necessary. Most of the time it is better to
pass the private driver pointer anyway.

Also refactor the code so that code that the driver model implementation
will share are available in functions that can be called. Add stubs where
necessary.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-on: Apalis T30 2GB on Apalis Evaluation Board