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.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/20240412115729.8316-3-pkshih@realtek.com