rtc: sd3078: make symbol 'sd3078_driver' static

Fixes the following sparse warning:

drivers/rtc/rtc-sd3078.c:218:19: warning:
 symbol 'sd3078_driver' was not declared. Should it be static?

Fixes: 1d67a23210 ("rtc: sd3078: new driver.")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-sd3078.c

@@ -215,7 +215,7 @@ static const struct of_device_id rtc_dt_match[] = {
 };
 MODULE_DEVICE_TABLE(of, rtc_dt_match);
 
-struct i2c_driver sd3078_driver = {
+static struct i2c_driver sd3078_driver = {
 	.driver     = {
 		.name   = "sd3078",
 		.owner  = THIS_MODULE,