sh: r2dplus: Enable board_eth_init only for non-DM case

The board_eth_init() is not used for DM case, enable it only for
the non-DM case. This function should eventually be removed.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>