libertas: use mesh_wdev->ssid instead of priv->mesh_ssid

With the commit e86dc1ca ("Libertas: cfg80211 support") we've lost
the ability to actually set the Mesh SSID from userspace.
NL80211_CMD_SET_INTERFACE with NL80211_ATTR_MESH_ID sets the mesh point
interface's ssid field. Let's use that one for the Libertas Mesh
operation

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>