rtc: efi: Avoid spamming the log on RTC read failure

There are cases where the EFI runtime services may end up in a funny
state, e.g., due to a crash in the variable services, and this affects
other EFI runtime services as well.

That means that, even though GetTime() should not return an error, there
are cases where it might, and there is no point in logging such an
occurrence multiple times.

This works around an issue where user space -apparently- keeps hitting
on /dev/rtc if it fails to read the h/w clock, resulting in a tsunami of
log spam and a non-responsive system as a result.

Cc: Pierre Gondois <pierre.gondois@arm.com>
Cc: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/all/Y2o1hdZK9GGDVJsS@monolith.localdoman/Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230217142338.1444509-1-ardb@kernel.orgSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-efi.c

@@ -164,7 +164,7 @@ static int efi_read_time(struct device *dev, struct rtc_time *tm)
 
 	if (status != EFI_SUCCESS) {
 		/* should never happen */
-		dev_err(dev, "can't read time\n");
+		dev_err_once(dev, "can't read time\n");
 		return -EINVAL;
 	}