Mtk_soc_eth watchdog timeout after r11573

@neheb: I am almost at 10 days uptime, and with this patch applied, there is no kernel errors at all. Since this issue was "introduced", it never happened before that I had a clean kernel log for 10 days. So it seems this patch actually fixes or at least eliminates this issue. Adding it to master would allow more people to test it on more platforms.