nl80211: don't expose wdev->ssid for most interfaces

For mesh, this is simply wrong - there's no SSID, only the
mesh ID, so don't expose it at all.
For (P2P) client, it's wrong, because it exposes an internal
value that's only used when certain APIs are used.
For AP, it's actually the only correct case, so leave that.
All other interface types shouldn't be setting this anyway,
so there it won't change anything.

Fixes: b84e7a05

 ("nl80211: send the NL80211_ATTR_SSID in nl80211_send_iface()")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>