wifi: mt76: mt7996: fix TWT issues

This patch fixes the following TWT issues:
- Change table_mask to u16 to support up to 16 TWT stations
- Reject TWT flows for duplicated establishment
- Fix possible unaligned pointer
- Remove unsupported TWT_CONTROL_WAKE_DUR_UNIT flag
- The minimum TWT duration supported by mt7996 chipsets is 64. Reply
  with TWT_SETUP_CMD_DICTATE if the min_twt_dur is smaller than 64

Fixes: 98686cd2

 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>