pcnet32: remove private net_device_stats structure

Remove the statistics from the private structure.
Use the net_device_stats in netn_device structure.

Following Jeff Garzik's massive cleanup Sep 01.
pcnet32 was not "low-hanging fruit".

Tested x86_64.

Signed-off-by: Don Fry <pcnet32@verizon.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>