watchdog: ziirave_wdt: Zero out only what's necessary

Instead of zeroing out all of the packet and then overwriting a
significant portion of those zeros via memcpy(), zero out only a
portion of the packet that is known to not contain any data.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Rick Ramstetter <rick@anteaterllc.com>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190812200906.31344-11-andrew.smirnov@gmail.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>