ramips: disable CONFIG_LIST_HARDENED on MT7620

ramips: disable CONFIG_LIST_HARDENED on MT7620

In #16396, crashes were reported on MT7620, which were introduced by enabling
CONFIG_LIST_HARDENED. The cause seems to be random memory corruption somewhere
in the kernel. Unfortunately the crash traces do not point to the real cause
of the crash in any way.
Since MT7620 is really ancient hardware that likely only has few users left,
I don't expect that anybody will invest a significant amount of time to track
down the real cause. Because of that, let's disable CONFIG_LIST_HARDENED on
this target only, and leave it enabled on all other platforms.

Fixes: #16396
Signed-off-by: Felix Fietkau <nbd@nbd.name>