1. Add phy_linkchange function to simplify eth_phy_isr and phy_monitor_thread_entry
2. phy_speed is uint8_t may case some problems, use enumeration instead of hardware-dependent values
3. Fix eth_phy_isr can not detect link up
4. Use macro to check phy status for flexibility
Signed-off-by: Nxiaofan <xfan1024@live.com>