watchdog: sch311x_wdt: Fix Polarity when starting watchdog

Some motherboards like the Advantech ARK3400 documentation
use a non-inverted GPIO pin. We fix this by assuming that
the BIOS will set the Polarity bit for the GPIO correctly
at startup and we keep the Bit-setting intact when we start
and stop the watchdog.

Reported-by: Jean-François Deverge <jf.deverge@gmail.com>
Signed-off-by: Dave Mueller <d.mueller@elsoft.ch>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>