rtc: nvmem: emit an error message when nvmem registration fails

Some users check the return value of devm_rtc_nvmem_register() only in
order to emit an error message and then continue probing. This is fine
as an rtc can function without exposing nvmem but let's generalize it:
let's make the registration function emit the error message so that
users don't have to.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201109163409.24301-7-brgl@bgdev.pl