watchdog: fix a small number of "watchog" typos in comments

All typos in comments, should not affect functionality.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
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>