staging: gdm72xx: zero out padding

We pad the start of this buffer with 256 bytes of padding.   It's not
clear to me exactly what's going on or how it's used but let's zero it
out.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>