watchdog: da9063: Fix setting/changing timeout

If the timeout value is set more than once the DA9063 watchdog triggers
a reset signal which reset the system.

To update the timeout value we have to disable the watchdog, clear the
watchdog counter value and write the new timeout value to the watchdog.
Clearing the counter value is a feature to be on the safe side because the
data sheet doesn't describe the behaviour of the watchdog counter value
after a watchdog disabling-enable-sequence.

The patch is based on Philipp Zabel's previous patch.

Fixes: 5e9c16e3 ("watchdog: Add DA9063 PMIC watchdog driver.")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
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>