p54: fix a NULL pointer dereference bug

If the RSSI calibration table was not found or not parsed properly,
priv->rssi_db will be NULL, p54_rssi_find needs to be able to deal
with that.
Signed-off-by: NFelix Fietkau <nbd@openwrt.org>
Acked-by: NChristian Lamparter <chunkeey@googlemail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/p54/eeprom.c

@@ -524,10 +524,13 @@ static int p54_parse_rssical(struct ieee80211_hw *dev,
 
 struct p54_rssi_db_entry *p54_rssi_find(struct p54_common *priv, const u16 freq)
 {
-	struct p54_rssi_db_entry *entry = (void *)(priv->rssi_db->data +
-						   priv->rssi_db->offset);
+	struct p54_rssi_db_entry *entry;
 	int i, found = -1;
 
+	if (!priv->rssi_db)
+		return &p54_rssi_default;
+
+	entry = (void *)(priv->rssi_db->data + priv->rssi_db->offset);
 	for (i = 0; i < priv->rssi_db->entries; i++) {
 		if (!same_band(freq, entry[i].freq))
 			continue;