libertas: don't write wdev->ssid/_len

When joining an IBSS network, wdev->ssid/_len will already be
set, so there's no need to write them. In any case, they are
internal cfg80211 values, and have very little user-visible
impact.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>