net: pcnet: Wrap devbusfn into private data

Instead of using eth_device priv for this PCI devbusfn, free it
so it could be used for driver private data, and wrap devbusfn
into those driver private data.

Note that using the name dev for the variable is a trick left for
later, when DM support is in place, so dm_pci_virt_to_mem() can be
used with minimal ifdeffery.

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