net: avoid eth_unregister() call when function is unavailable

net: avoid eth_unregister() call when function is unavailable

CONFIG_NETCONSOLE causes common/bootm.c to call eth_unregister()
for network device shutdown. However, with CONFIG_DM_ETH this
function is no longer defined.

This is a workaround to avoid the call in that case, and solely
rely on eth_halt(). In case this is insufficient, a proper way
to unregister / remove network devices needs to be implemented.

Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>