Watchdog: remove outdated comment

The lock field doesn't exist in watchdog_device structure.
It was added by commit f4e9c82f ("watchdog: Add Locking support")
and removed by commit b4ffb190
("watchdog: Separate and maintain variables based on variable lifetime")

Signed-off-by: Hardik Singh Rathore <hardiksingh.k@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>