wifi: mt76: mt792x: fix a potential loading failure of the 6Ghz channel config from ACPI

In some case, the MTCL table will exist, but MTDS table will not.
So the SAR will init fail. This patch make MTCL and MTDS can exist
with no dependence.

Fixes: f965333e ("mt76: mt7921: introduce ACPI SAR support")
Signed-off-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Signed-off-by: Leon Yen <leon.yen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>