wl12xx: Set IEEE80211_TX_RC_SHORT_GI if short GI was used on

New FW reports usage of short GI as a rate class index. Check for
this rate and set the IEEE80211_TX_RC_SHORT_GI if used.

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