wifi: mt76: mt7921: fix a potential clc buffer length underflow

wifi: mt76: mt7921: fix a potential clc buffer length underflow

The buf_len is used to limit the iterations for retrieving the country
power setting and may underflow under certain conditions due to changes
in the power table in CLC.

This underflow leads to an almost infinite loop or an invalid power
setting resulting in driver initialization failure.

Cc: stable@vger.kernel.org
Fixes: fa6ad88e023d ("wifi: mt76: mt7921: fix country count limitation for CLC")
Signed-off-by: Leon Yen <leon.yen@mediatek.com>
Signed-off-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Link: https://patch.msgid.link/20251009020158.1923429-1-mingyen.hsieh@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>