rtw88: fix unassigned rssi_level in rtw_sta_info

The new rssi_level should be stored in si, otherwise the rssi_level will
never be updated and get a wrong RA mask, which is calculated by the
rssi level

If a wrong RA mask is chosen, the firmware will pick some *bad rates*.
The most hurtful scene will be in *noisy environment*, such as office or
public area with many APs and users.
The latency would be high and the overall throughput would be only half
or less.

Tested in 2.4G in office area, with this patch the throughput increased
from such as "1x Mbps -> 4x Mbps".

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>