rtc: pcf2123: convert to devm_rtc_allocate_device

This allows further improvement of the driver. Also remove the unecessary
error string as the core will already display error messages.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20190819182656.29744-7-alexandre.belloni@bootlin.com


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