net: Move remaining common functions to eth_common.c

Move eth_current_changed(), eth_set_current(), eth_mac_skip() and
eth_get_name() into the common file.

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