watchdog: watchdog_dev: Use single variable name for struct watchdog_device

The current code uses 'wdd', wddev', and 'watchdog' as variable names
for struct watchdog_device. This is confusing and makes it difficult
to enhance the code. Replace it all with 'wdd'.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: Timo Kokkonen <timo.kokkonen@offcode.fi>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>