rtc: efi: Fail probing if RTC reads don't work

While the EFI spec mandates an RTC, not every implementation actually adheres
to that rule (or can adhere to it - some systems just don't have an RTC).

For those, we really don't want to probe the EFI RTC driver at all, because if
we do we'd get a non-functional driver that does nothing useful but only spills
our kernel log with warnings.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>