net: sh_eth: Add R8A77980 V3H gether support

The R8A77980 V3H gether needs a few minor adjustments to the sh_eth
driver, add them to support ethernet on R8A77980.

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