watchdog: kempld-wdt: Use the correct value when configuring the prescaler with the watchdog

Use the prescaler index, rather than its value, to configure the watchdog.
This will prevent a mismatch with the prescaler used to calculate the cycles.

Signed-off-by: Per Gundberg <per.gundberg@icomera.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Michael Brunner <michael.brunner@kontron.com>
Tested-by: Michael Brunner <michael.brunner@kontron.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: stable <stable@vger.kernel.org>