wifi: mt76: mt7996: handle IEEE80211_RC_SMPS_CHANGED

Make mt7996_mcu_set_fixed_field() non-static in order to handle
IEEE80211_RC_SMPS_CHANGED in mt7996_mac_sta_rc_work().

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>