ath6kl: fix link speed when using sgi

The MSB of rate index from FW is used for sgi. But the ath6kl_wmi_get_rate
doesn't handle it. The access to wmi_rate_tbl array may be out of range
if sgi is 1. This may cause the return value of ath6kl_wmi_get_rate()
function is incorrect link rate. We add sgi adjustment to avoid such case.

kvalo: change patch title

Signed-off-by: Dengke Qiu <dqiu@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>