generic: mtk_eth_soc: fix PPE hanging issue

generic: mtk_eth_soc: fix PPE hanging issue

A patch to resolve an issue was found in MediaTek's GPL-licensed SDK:
In the mtk_ppe_stop() function, the PPE scan mode is not disabled before
disabling the PPE. This can potentially lead to a hang during the process
of disabling the PPE.

Without this patch, the PPE may experience a hang during the reboot test.

Reference: https://git01.mediatek.com/plugins/gitiles/openwrt/feeds/mtk-openwrt-feeds/+/b40da332dfe763932a82f9f62a4709457a15dd6c

Suggested-by: Bc-bocun Chen <bc-bocun.chen@mediatek.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 003b9ff61c081dd820764d4d0a83bb2514646576)