mt76: mt7915: add twt_stats knob in debugfs

Introduce twt_stats knob in debugfs in order to dump established
agreements

Tested-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Tested-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>