USB: usbmon: remove assignment from IS_ERR argument

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

)

// <smpl>
@@
expression e1,e2;
statement S1,S2;
@@

+e1 = e2;
if (IS_ERR(
    e1
-   = e2
   )) S1 else S2
// </smpl>

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