rtc: sunplus: fix return value in sp_rtc_probe()

If devm_ioremap_resource() fails, it should return error
code from sp_rtc->reg_base in sp_rtc_probe().

Fixes: fad6cbe9

 ("rtc: Add driver for RTC in Sunplus SP7021")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220106075711.3216468-1-yangyingliang@huawei.com