net: wifi: Unicast frames statistics

Unicast RX stats
It represents total number of unicast (any type data, action or
any other unicast frames) frames received at firmware level.
The actual frames passed to host will be different as firmware
may drop packets or some packets may be dropped because of
errors.

Unicast TX stats
Transmission side the unicast packets count states the packets
handed over to firmware. The stats taken at firmware level.
Actual packets transmission may vary depending upon various factors.

Signed-off-by: Ajay Parida <ajay.parida@nordicsemi.no>