mt76: mt7915: reject duplicated twt flows

Reject twt flows with the same parameters to prevent some potential issues
causing by duplicated establishment.

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>