wl12xx: move tx_security_seq into wlvif

The last security seq num has to be saved across reconfigs.
Add a new "persistent" struct into wlvif, which won't get
deleted on wl12xx_init_vif_data()

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>