pcnet: pcnet_common_init() always returns 0, change to void

The next commit will exploit the fact it never fails.  This one makes
it obvious.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>


