rtc: rv8803: fix missing unlock on error in rv8803_set_time()

Add the missing unlock before return from function rv8803_set_time()
in the error handling case.

Fixes: f8176e0b ("rtc: rv8803: initialize registers on post-probe voltage loss")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220627080822.3881072-1-yangyingliang@huawei.com

drivers/rtc/rtc-rv8803.c

@@ -315,8 +315,10 @@ static int rv8803_set_time(struct device *dev, struct rtc_time *tm)
 
 	if (flags & RV8803_FLAG_V2F) {
 		ret = rv8803_regs_reset(rv8803);
-		if (ret)
+		if (ret) {
+			mutex_unlock(&rv8803->flags_lock);
 			return ret;
+		}
 	}
 
 	ret = rv8803_write_reg(rv8803->client, RV8803_FLAG,