Replace WLAN_802_11_SSID with direct 'ssid' and 'ssid_len' members
like ieee80211.  In the process, remove private libertas_escape_essid
and depend on the ieee80211 implementation of escape_essid instead.
Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>