brcmfmac: support CQM RSSI notification with older firmware

Using the BCM4339 firmware from linux-firmware (version "BCM4339/2 wl0:
Sep  5 2019 11:05:52 version 6.37.39.113 (r722271 CY)" from
cypress/cyfmac4339-sdio.bin) the RSSI respose is only 4 bytes, which
results in an error being logged.

It seems that older devices send only the RSSI field and neither SNR nor
noise is included.  Handle this by accepting a 4 byte message and
reading only the RSSI from it.

Fixes: 7dd56ea ("brcmfmac: add support for CQM RSSI notifications")
Signed-off-by: John Keeping <john@metanate.com>

drivers/net/wireless/broadcom/brcm80211/brcmfmac/cfg80211.c

@@ -6489,18 +6489,20 @@ static s32 brcmf_notify_rssi(struct brcmf_if *ifp,
 {
 	struct brcmf_cfg80211_vif *vif = ifp->vif;
 	struct brcmf_rssi_be *info = data;
-	s32 rssi, snr, noise;
+	s32 rssi, snr = 0, noise = 0;
 	s32 low, high, last;
 
-	if (e->datalen < sizeof(*info)) {
+	if (e->datalen >= sizeof(*info)) {
+		rssi = be32_to_cpu(info->rssi);
+		snr = be32_to_cpu(info->snr);
+		noise = be32_to_cpu(info->noise);
+	} else if (e->datalen >= sizeof(rssi)) {
+		rssi = be32_to_cpu(*(s32 *)data);
+	} else {
 		brcmf_err("insufficient RSSI event data\n");
 		return 0;
 	}
 
-	rssi = be32_to_cpu(info->rssi);
-	snr = be32_to_cpu(info->snr);
-	noise = be32_to_cpu(info->noise);
-
 	low = vif->cqm_rssi_low;
 	high = vif->cqm_rssi_high;
 	last = vif->cqm_rssi_last;