drivers: rtc: maxim,ds3231: avoid compilation error

Because of wrong integer type syntax following error occurred:

drivers/rtc/rtc_ds3231.c:361:76: error: unknown type name 'u'
  361 | static int rtc_ds3231_alarm_get_supported_fields(
      |                    const struct device *dev, u int16_t id,
      |                                              ^

Signed-off-by: Stephan Linz <linz@li-pro.net>