ath10k: fix the wrong RX rate idx report at 11G mode

The RX rate idx is not correct for 11G mode OFDM packet.
Because the bitrate table start with CCK index instead of OFDM.

Signed-off-by: Yanbo Li <yanbol@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>