rtc: Use dev_get_drvdata()

Using dev_get_drvdata directly.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: linux-rtc@vger.kernel.org
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>