n_gsm: Drop unneeded cast on netdev_priv

The result of netdev_priv is already implicitly cast to the type of the
left side of the assignment.

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

)

// <smpl>
@@
type T;
T *x;
@@

x =
- (T *)
  netdev_priv(...)
// </smpl>

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