wifi: rtw89: avoid stringop-overflow warning

After -Wstringop-overflow got enabled, the rtw89 driver produced
two odd warnings with gcc-13:

drivers/net/wireless/realtek/rtw89/coex.c: In function 'rtw89_btc_ntfy_scan_start':
drivers/net/wireless/realtek/rtw89/coex.c:5362:50: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
 5362 |                 wl->dbcc_info.scan_band[phy_idx] = band;
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~
In file included from drivers/net/wireless/realtek/rtw89/coex.h:8,
                 from drivers/net/wireless/realtek/rtw89/coex.c:5:
drivers/net/wireless/realtek/rtw89/core.h:1441:12: note: at offset [64, 255] into destination object 'scan_band' of size 2
 1441 |         u8 scan_band[RTW89_PHY_MAX]; /* scan band in  each phy */
      |            ^~~~~~~~~
drivers/net/wireless/realtek/rtw89/coex.c: In function 'rtw89_btc_ntfy_switch_band':
drivers/net/wireless/realtek/rtw89/coex.c:5406:50: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
 5406 |                 wl->dbcc_info.scan_band[phy_idx] = band;
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~
drivers/net/wireless/realtek/rtw89/core.h:1441:12: note: at offset [64, 255] into destination object 'scan_band' of size 2
 1441 |         u8 scan_band[RTW89_PHY_MAX]; /* scan band in  each phy */
      |            ^~~~~~~~~

I don't know what happened here, but adding an explicit range check
shuts up the output.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231204073020.1105416-1-arnd@kernel.org

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

@@ -5352,6 +5352,10 @@ void rtw89_btc_ntfy_scan_start(struct rtw89_dev *rtwdev, u8 phy_idx, u8 band)
 	rtw89_debug(rtwdev, RTW89_DBG_BTC,
 		    "[BTC], %s(): phy_idx=%d, band=%d\n",
 		    __func__, phy_idx, band);
+
+	if (phy_idx >= RTW89_PHY_MAX)
+		return;
+
 	btc->dm.cnt_notify[BTC_NCNT_SCAN_START]++;
 	wl->status.map.scan = true;
 	wl->scan_info.band[phy_idx] = band;
@@ -5396,6 +5400,10 @@ void rtw89_btc_ntfy_switch_band(struct rtw89_dev *rtwdev, u8 phy_idx, u8 band)
 	rtw89_debug(rtwdev, RTW89_DBG_BTC,
 		    "[BTC], %s(): phy_idx=%d, band=%d\n",
 		    __func__, phy_idx, band);
+
+	if (phy_idx >= RTW89_PHY_MAX)
+		return;
+
 	btc->dm.cnt_notify[BTC_NCNT_SWITCH_BAND]++;
 
 	wl->scan_info.band[phy_idx] = band;