net: pcnet: Wrap name and enetaddr into private data

Instead of using the non-DM-only name and enetaddr in struct eth_device,
add pointers into the private data which can either point to that non-DM
name or a DM one later on.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>