wifi: rtw89: fw: scan offload prohibit all 6 GHz channel if no 6 GHz sband

In the Linux kernel, the following vulnerability has been resolved: wifi: rtw89: fw: scan offload prohibit all 6 GHz channel if no 6 GHz sband We have some policy via BIOS to block uses of 6 GHz. In this case, 6 GHz sband will be NULL even if it is WiFi 7 chip. So, add NULL handling here to avoid crash.