net: sh_eth: Split sh_eth_init

Split sh_eth_init() function into smaller chunks, which can
be called from both DM and non-DM code while handling the
specifics of both configurations.

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