watchdog: ep93xx_wdt: Fix timeout after conversion to watchdog core

After the conversion of this driver to the watchdog core, I noticed that we
miss setting the initial timeout of the wdt device.
This results in a failure of the WDIOC_GETTIMEOUT ioctl call.

Reviewed-by: Mika Westerberg <mika.westerberg@iki.fi>
Tested-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>