rtc: nvmem: don't return an error when not enabled

Avoid reporting an error when RTC_NVMEM is not selected.

Reported-by: kernel test robot <xiaolong.ye@intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index 4c007f6..6268208 100644
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -285,7 +285,7 @@
 static inline int rtc_nvmem_register(struct rtc_device *rtc,
 				     struct nvmem_config *nvmem_config)
 {
-	return -ENODEV;
+	return 0;
 }
 static inline void rtc_nvmem_unregister(struct rtc_device *rtc) {}
 #endif