watchdog: imx_sc_wdt: Pretimeout should follow SCU firmware format

SCU firmware calculates pretimeout based on current time stamp
instead of watchdog timeout stamp, need to convert the pretimeout
to SCU firmware's timeout value.

Fixes: 15f7d7fc

 ("watchdog: imx_sc: Add pretimeout support")
Signed-off-by: Anson Huang <Anson.Huang@nxp.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>