rtw89: handle potential uninitialized variable

The smatch reports:
 rtw8852a.c:1857 rtw8852a_btc_set_wl_txpwr_ctrl() error: uninitialized symbol '_cur'.
 rtw8852a.c:1858 rtw8852a_btc_set_wl_txpwr_ctrl() error: uninitialized symbol '_cur'.

This is because rtw89_mac_txpwr_read32() can possibly return before setting
argument _cur, and the caller will use the uninitialized value. To fix this
problem, check the return value of rtw89_mac_txpwr_read32().
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220318035202.42437-1-pkshih@realtek.com

drivers/net/wireless/realtek/rtw89/rtw8852a.c

@@ -1843,7 +1843,8 @@ rtw8852a_btc_set_wl_txpwr_ctrl(struct rtw89_dev *rtwdev, u32 txpwr_val)
 		u32 _cur, _wrt;						\
 		rtw89_debug(rtwdev, RTW89_DBG_TXPWR,			\
 			    "btc ctrl %s: 0x%x\n", #_case, _val);	\
-		rtw89_mac_txpwr_read32(rtwdev, RTW89_PHY_0, _reg, &_cur);\
+		if (rtw89_mac_txpwr_read32(rtwdev, RTW89_PHY_0, _reg, &_cur))\
+			break;						\
 		rtw89_debug(rtwdev, RTW89_DBG_TXPWR,			\
 			    "btc ctrl ori 0x%x: 0x%x\n", _reg, _cur);	\
 		_wrt = __do_clr(_val) ?					\