[WATCHDOG] Documentation/watchdog/src/watchdog-simple.c: improve this code

Make some improvements for Documentation/watchdog/src/watchdog-simple.c.

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>