Restore preemphasis when AN is transitioned from ON to OFF

[PJHsieh]

What I did Restore preemphasis when AN is transitioned from ON to OFF

Why I did it In certain switches, link training is enabled along with auto-negotiation(AN) enabled. In this case, the preemphasis settings may be changed due to the presence of enabled link training. Therefore, it's essential to restore the preemphasis value when auto-negotiation transitions from ON to OFF.

How I verified it

1. Check the current pre-emphasis value P1 by bcmcmd.
2. Enable AN on a port, make sure the port is up and the pre-emphasis is changed to P2 by link-training.
3. Disable AN on the testing port, check the pre-emphasis value is restored to P1 by bcmcmd.

Details if related

[PJHsieh]

Hi, @prsunny ,

Please review this PR, thank you.