watchdog: f71808e_wdt: fix F81866 bit operation

Fix error bit operation in watchdog_start()

Fixes: 14b24a88 ("watchdog: f71808e_wdt: Add F81866 support")
Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@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>