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.cnSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-rv3028.c

@@ -521,10 +521,9 @@ static int rv3028_param_get(struct device *dev, struct rtc_param *param)
 {
 	struct rv3028_data *rv3028 = dev_get_drvdata(dev);
 	int ret;
+	u32 value;
 
 	switch(param->param) {
-		u32 value;
-
 	case RTC_PARAM_BACKUP_SWITCH_MODE:
 		ret = regmap_read(rv3028->regmap, RV3028_BACKUP, &value);
 		if (ret < 0)
@@ -554,9 +553,9 @@ static int rv3028_param_get(struct device *dev, struct rtc_param *param)
 static int rv3028_param_set(struct device *dev, struct rtc_param *param)
 {
 	struct rv3028_data *rv3028 = dev_get_drvdata(dev);
+	u8 mode;
 
 	switch(param->param) {
-		u8 mode;
 	case RTC_PARAM_BACKUP_SWITCH_MODE:
 		switch (param->uvalue) {
 		case RTC_BSM_DISABLED: