rtc: rv3028: Fix codestyle errors

rtc: rv3028: Fix codestyle errors

Compiler warnings：

drivers/rtc/rtc-rv3028.c: In function 'rv3028_param_set':
drivers/rtc/rtc-rv3028.c:559:20: warning: statement will never be executed [-Wswitch-unreachable]
  559 |                 u8 mode;
      |                    ^~~~
drivers/rtc/rtc-rv3028.c: In function 'rv3028_param_get':
drivers/rtc/rtc-rv3028.c:526:21: warning: statement will never be executed [-Wswitch-unreachable]
  526 |                 u32 value;
      |                     ^~~~~

Fix it by moving the variable declaration to the beginning of the function.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: linux-rtc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: Ke Sun <sunke@kylinos.cn>
Link: https://lore.kernel.org/r/20221008071321.1799971-1-sunke@kylinos.cn
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>