net: tulip: delete useless tests on netdev_priv

Netdev_priv performs an addition, not a pointer dereference, so it seems
quite unlikely that its result would ever be NULL.

A semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
statement S;
@@

- if (!netdev_priv(...)) S
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>