staging: et131x: Remove useless assignment to NULL

The stack variable skb is no longer used after it's set to
NULL. Don't set it to NULL.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>