wifi: mt76: mt7996: add full system reset knobs into debugfs

Add testing points into debugfs to trigger firmware assert and enable
full system recovery. Also rename knob "fw_ser" to a clear-cut name
"sys_recovery".

Co-developed-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>