We need wdev_chandef() in this code, which now requires
the wdev mutex due to the per-link nature. Hold it here
to make sure we can access the link.

Reported-by: syzbot+b4e9aa0f32ffd9902442@syzkaller.appspotmail.com
Fixes: 7b0a0e3c ("wifi: cfg80211: do some rework towards MLO link APIs")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>