net: pcnet: Pass private data through dev->priv

Get rid of the global point to private data, and rather pass it
thought dev->priv. Also remove the unnecessary check for lp being
non-NULL, since it is always NULL at this point.

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