net: pcnet: Wrap iobase into private data

Instead of using the non-DM-only iobase in struct eth_device, add
one into the private data to make DM and non-DM operation possible.

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