net: Move common init into a new eth_common.c file

Only half of the init is actually common. Move that part into a new common
file and call it from driver-model and legacy code. More common functions
will be added in future patches.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>