net: eth: Clear MAC address in eth_pre_remove()

platdata->enetaddr was assigned to a value in dev_probe() last time.
If we don't clear it, for dev_probe() at the second time, dm eth
will end up treating it as a MAC address from ROM no matter where it
came from originally (maybe env, ROM, or even random). Fix this by
clearing platdata->enetaddr when removing an Ethernet device.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>