[TG3]: Some low power fixes

Add some missing workarounds in tg3_set_power_state():

1. Workaround to prevent overdrawing current on 5714.

2. Do not power down 5700's PHY because of hw limitation.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>