watchdog: WatchDog Timer Driver Core - use passed watchdog_device

Use the passed watchdog_device instead of the static global variable when
testing and setting the status in watchdog_ping, watchdog_start, and
watchdog_stop.  Note that the callers of these functions are actually
passing the static global variable.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>