wifi: mt76: mt7925: fix tx power setting failure after chip reset

wifi: mt76: mt7925: fix tx power setting failure after chip reset

After the chip reset, the procedure to set the tx power will not be
successful because the previous region setting is still remains.
Clear the region setting during MAC initialization and allow it to be
reset to finalize the TX power setting.

Fixes: 3bc62aa4484d ("wifi: mt76: mt7925: add auto regdomain switch support")
Signed-off-by: Leon Yen <leon.yen@mediatek.com>
Link: https://patch.msgid.link/20260120163152.3694116-1-leon.yen@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>