rtc: nct3018y: fix possible NULL dereference

alarm_enable and alarm_flag are allowed to be NULL but will be dereferenced
later by the dev_dbg call.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202305180042.DEzW1pSd-lkp@intel.com/
Link: https://lore.kernel.org/r/20240229222127.1878176-1-alexandre.belloni@bootlin.com


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