watchdog: wdt285: Fix variable type

Variable "new_margin" is checked in the function watchdog_ioctl()
to be non-negative, so change its type to "int".

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>