wifi: mt76: mt7996: enable full system reset support

Add mt7996_reset() and refactor mt7996_mac_reset_work() to support
full system recovery.

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>