rtc: cros-ec: remove superfluous error message

The RTC core now has error messages in case of registration failure, there
is no need to have other messages in the drivers.

Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20191016201414.30934-1-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>