[U-Boot] 83xx, uec: adjust enet_interface settings on the fly.

[Liu Dave-R63238]

> did you make any effort to refactor this into the existing 
> eth_type == GIGA_ETH code?  I'm not sure why that code became 
> conditional in commit 
> 24c3aca3f1358b113d3215adb5433b156e99f72b "mpc83xx: Add 
> support for the MPC832XEMDS board" in the first place - Dave?

I don't remember why I added the eth_type==GIGA_ETH condition.
If it is possible, please refactor it as Kim.

Thanks, Dave