wifi: mt76: mt7921: move mt7921u_disconnect mt792x-lib

mt7921u_disconnect routine is shared between mt7921 and mt7925 so move
it in mt792x-usb module.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>