nl80211: send the NL80211_ATTR_SSID in nl80211_send_iface()

The userspace may want to know what is the current ssid that a given
interface is using. This patch enables nl80211 to send the
NL80211_ATTR_SSID attribute in nl80211_send_iface().

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>