ath10k: fix rssi and rate reporting

RSSI and RATES fields are valid only when START_VALID
bit is set. So, in current implementation we have to
remember/caclulate them when START_VALID and report the same
when only END_VALID is set.
Currently during heavy traffic we could have:
- 10 packets with START_VALID - correct RSSI and RATES
- 10 packets with END_VALID
- 10 packets with START_VALID - correct RSSI and RATES
- 10 packets with END_VALID
...
Next using monitor interface we will see:
- 10 packets with correct rssi/rates
- 10 packets with rssi=-95/rate=6Mbps

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>