wifi: mac80211: Fix permissions for valid_links debugfs entry

The entry should be a read only one and not a write only one. Fix it.

Fixes: 3d901102 ("wifi: mac80211: implement link switching")
Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230611121219.c75316990411.I1565a7fcba8a37f83efffb0cc6b71c572b896e94@changeid


[remove x16 change since it doesn't work yet]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>