wifi: rtw89: regd: block 6 GHz if marked as N/A in regd map

If 6 GHz of a country is marked as N/A in our regd map,
we block 6 GHz channels now.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20241016134457.9375-1-pkshih@realtek.com