nl80211: add NL80211_FLAG_NEED_WDEV

Some nl80211 callbacks will soon need the wdev instead
of the netdev, so add NL80211_FLAG_NEED_WDEV to allow
them to request that. Add NL80211_FLAG_NEED_WDEV_UP as
well which checks the netdev is UP if one exists.

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