rtc: stm32: don't print an error on probe deferral

Change stm32-rtc driver to not generate an error message when
device probe operation is deferred for a clock.

Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20230705174357.353616-5-valentin.caron@foss.st.com


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