rtc: class: add debug message when registration fails

Add a message when __rtc_register_device fails because rtc->ops is NULL.
This may only happen when developing a new driver so use dev_dbg to avoid
compiling it in by default.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20190818220041.17833-1-alexandre.belloni@bootlin.com


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