staging: r8712u: delete unnecessary field initialization

On success, the function netdev_alloc_skb initializes the dev field of its
result to its first argument, so this doesn't have to be done in the
calling context.

The semantic patch that fixes this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression skb,privn,e;
@@

skb = netdev_alloc_skb(privn,...);
... when strict
(
-skb->dev = privn;
|
?skb = e
)
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>