libertas: don't depend on IEEE80211

Runtime-wise we only need escape_ssid from the deprecated IEEE80211
subsystem. However, it's easy to provide our own copy.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>