drivers: sensor: maxim,ds3231: avoid compilation error

As the C language treats 'cases' similar to 'labels' the following
error can also occur when using switch statements when it encounters
a declaration immediately after a label:

drivers/sensor/maxim/ds3231/ds3231.c:77:3: error: a label can only
        be part of a statement and a declaration is not a statement
   77 |   const uint16_t raw_temp = data->raw_temp;
      |   ^~~~~

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