rtlwifi: rtl8723be: avoid undefined behavior

Do not return undefined value for transmission power
if the rate is invalid.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtlwifi/rtl8723be/phy.c

@@ -1019,7 +1019,7 @@ static u8 _rtl8723be_get_txpower_index(struct ieee80211_hw *hw, u8 path,
 	struct rtl_priv *rtlpriv = rtl_priv(hw);
 	struct rtl_efuse *rtlefuse = rtl_efuse(rtl_priv(hw));
 	u8 index = (channel - 1);
-	u8 txpower;
+	u8 txpower = 0;
 	u8 power_diff_byrate = 0;
 
 	if (channel > 14 || channel < 1) {