wifi: rtl8xxxu: Report the signal strength only if it's known

wifi: rtl8xxxu: Report the signal strength only if it's known

These chips don't report the signal strength for many (any?) data
frames. When the signal strength is not known, set RX_FLAG_NO_SIGNAL_VAL
in order to avoid reporting a signal strength of 0.

Tested with RTL8192FU and RTL8192CU.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/cdbe1a18-f6f1-4575-bad9-e85db8101247@gmail.com