arm64: dts: rockchip: fix rk3328-roc-cc gmac2io stability issues

This patch is a port of the fix from
commit 73e42e18 ("arm64: dts: rockchip: fix rock64 gmac2io stability
issues")

As per that patch, enabling thresh dma mode force disables checksuming.
This is necessary as tx checksuming does not work with packets larger
than 1498.

The rk3328-roc-cc board exhibits tx stability issues with large packets
similar to rock64's issues. This patch resolves that issue.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>