mac80211: remove warning in ieee80211_get_sband()

Syzbot reports that it's possible to hit this from userspace,
by trying to add a station before any other connection setup
has been done. Instead of trying to catch this in some other
way simply remove the warning, that will appropriately reject
the call from userspace.

Reported-by:  <syzbot+7716dbc401d9a437890d@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20210517164715.f537da276d17.Id05f40ec8761d6a8cc2df87f1aa09c651988a586@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>