mt76: mt7915: fix SMPS operation fail

TGn fails sending SM power save mode action frame to the AP to switch
from dynamic SMPS mode to static mode.

Reported-by: Fang Zhao <fang.zhao@mediatek.com>
Signed-off-by: Fang Zhao <fang.zhao@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>