wifi: mt76: mt7996: align the format of fixed rate command

Use the new fixed rate command format to let the fixed field function
work normally.

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>