wl12xx: Set IEEE80211_TX_RC_MCS on MCS rates on TX complete.

IEEE80211_TX_RC_MCS was not set correctly leading to incorrect link
speed calculation.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: Ido Reis <idor@ti.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>