wifi: mt76: mt7921: add auto regdomain switch support

wifi: mt76: mt7921: add auto regdomain switch support

Implement 802.11d-based automatic regulatory domain switching to
dynamically determine the regulatory domain at runtime.

The scan-done event structure by reusing reserved padding and appending
new fields; the layout and values remains backward-compatible with
existing users.

Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: JB Tsai <jb.tsai@mediatek.com>
Link: https://patch.msgid.link/20260303053637.465465-4-jb.tsai@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>