wifi: mt76: mt7915: enable full system reset support

Add mt7915_reset() and refactor mt7915_mac_reset_work() to support
full system recovery.

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