mt76: mt7915: fix twt table_mask to u16 in mt7915_dev

mt7915 can support 16 twt stations so modify table_mask to u16.

Fixes: 3782b69d ("mt76: mt7915: introduce mt7915_mac_add_twt_setup routine")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>