dm: eth: Stick to 'ethact' when 'ethrotate' is 'no' in eth_init()

When 'ethrotate' variable is set to 'no' and 'ethact' variable
is already set to an ethernet device, we should stick to 'ethact'.

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