ath9k: make sure main_rssi is positive

some times the rssi control descriptor for the main antenna may be
negative like that of alternate antenna, hence before  incrementing
packet counts/rssi of main/alternate antenna make sure both main_rssi
and alt_rssi are positive only. this avoids wrong selection of antenna
due to diversity

Cc: Gabriel Tseng <Gabriel.Tseng@Atheros.com>
Cc: Senthilkumar Balasubramanian <Senthilkumar.Balasubramanian@Atheros.com>
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/recv.c

@@ -1468,8 +1468,8 @@ static void ath_ant_comb_scan(struct ath_softc *sc, struct ath_rx_status *rs)
 	main_ant_conf = (rs->rs_rssi_ctl2 >> ATH_ANT_RX_MAIN_SHIFT) &
 			 ATH_ANT_RX_MASK;
 
-	/* Record packet only when alt_rssi is positive */
-	if (alt_rssi > 0) {
+	/* Record packet only when both main_rssi and  alt_rssi is positive */
+	if (main_rssi > 0 && alt_rssi > 0) {
 		antcomb->total_pkt_count++;
 		antcomb->main_total_rssi += main_rssi;
 		antcomb->alt_total_rssi  += alt_rssi;