watchdog: da9063: rename helper function to avoid misunderstandings

_da9063_wdt_set_timeout() is called by da9063_wdg_set_timeout(),
da9063_wdg_start() and da9063_wdg_probe() but the name expect only to be
called by da9063_wdg_set_timeout(). Rename the function to avoid
misunderstandings.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Acked-by: Steve Twiss <stwiss.opensource@diasemi.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>