watchdog: Make API functions const correct

watchdog: Make API functions const correct

Many watchdog API functions do not modify the watchdog_device nor
device. Mark their arguments as const to reflect this and improve
const-correctness of the API.

No functional change intended.

Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>