dm: eth: Test 'ethrotate' before changing current ethernet device

In eth_current_changed(), the call to eth_get_dev() below has a side
effect of rotating ethernet device if uc_priv->current == NULL. This
is not what we want when 'ethrotate' variable is 'no'.

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>