watchdog: core: Fix devres_alloc() allocation size

Coverity reports:

Passing argument 152UL /* sizeof (*wdd) */ to function __devres_alloc_node
and then casting the return value to struct watchdog_device ** is
suspicious.

Allocation size needs to be sizeof(*rcwdd), not sizeof(*wdd).

Fixes: 83fbae5a ("watchdog: Add a device managed API for ...")
Cc: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>