drivers: rtc: maxim,ds3231: avoid warnings about unused code

Compile warnings occurred in common standard use case
w/o RTC alarm and update.

Warning was:

drivers/rtc/rtc_ds3231.c: In function 'rtc_ds3231_init':
drivers/rtc/rtc_ds3231.c:808:33: warning: unused variable 'data'
                 [-Wunused-variable]
  808 |         struct rtc_ds3231_data *data = dev->data;
      |                                 ^~~~
drivers/rtc/rtc_ds3231.c: At top level:
drivers/rtc/rtc_ds3231.c:195:12:
        warning: 'rtc_ds3231_get_ctrl_sts' defined but not used
                 [-Wunused-function]
  195 | static int rtc_ds3231_get_ctrl_sts(const struct device *dev,
      |                                    uint8_t *buf)
      |            ^~~~~~~~~~~~~~~~~~~~~~~

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