net: mediatek: connect switch to PSE only when starting eth is requested

So far the switch is initialized in probe stage and is connected to PSE
unconditionally. This will cause all packets being flooded to PSE and may
cause PSE hang before entering linux.

This patch changes the connection between switch and PSE:
- Still initialize switch in probe stage, but disconnect it with PSE
- Connect switch with PSE on eth start
- Disconnect on eth stop

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>