wl->vif is appropriate only when a single vif is being used.
Instead, pass wlvif as parameter or iterate through all
the vifs (e.g. when a global configuration was changed)

Leave wl->vif only to determine whether a vif was already
added (this check will be removed as well after both the
driver and fw will support multiple vifs)
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>