net: Divided code of NE2000 ethernet driver

There are more devices of the NE2000 base.
A present code is difficult for us to support more devices.
To support more NE2000 clone devices, separated the function.

Signed-off-by: Yusuke Goda <goda.yusuke@renesas.com>
Acked-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>