net: sh_eth: Clump legacy functions together

Move the legacy functions around, so that they can be wrapped in a
massive ifdef CONFIG_DM_ETH once DM support is added. No functional
change.

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