wl12xx: report invalid TX rate when returning non-TX-ed skbs

Report a TX rate idx of -1 and count 0 when returning untransmitted
skbs to mac80211 using ieee80211_tx_status(). Otherwise mac80211
tries to use the returned (essentially garbage) status.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>