staging: visornic: use skb_put_zero() instead of open-coded version

Replace an open-coded version of skb_put_zero() with a call to the helper.
This is a cleanup and is also useful for potentially adding KASAN
integration to SKBs in the future.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>