wifi: mt76: dynamic channel bandwidth changes in AP mode

Allow AP to change channel width for 40Mhz intolerant STA on the
2.4 GHz band.

Signed-off-by: Himanshu Goyal <himanshu.goyal@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>