rtc: fix RTC removal

Since the rtc_register_device, removing an RTC device will end with a
refcount_t: underflow; use-after-free warning since put_device is called
twice in the device tear down path.

Fixes: fdcfd854 ("rtc: rework rtc_register_device() resource management")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20201205231449.610980-1-alexandre.belloni@bootlin.com