net: sync env ethaddr to device enetaddr in eth_init()

In the previous enetaddr refactoring, the assumption with commit 56b555a6
was that the eth layer would handle the env -> device enetaddr syncing.
This was not the case as eth_initialize() is called only once and the sync
occurs there.  So make sure the eth_init() function does the env -> device
sync with every network init.

Reported-by: Andrzej Wolski <awolski@poczta.fm>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>