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: 1d67a232

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