rtc: rx8010: don't use magic values for time buffer length

The time buffer len is used directly in this driver. For readability
it's better to define it as the difference between the date register
offsets and use sizeof() whenever referencing it.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200914154601.32245-8-brgl@bgdev.pl