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