rtc: rx8010: prefer sizeof(*val) over sizeof(struct type_of_val)

Using the size of the variable is preferred over using the size of its
type when allocating memory. Convert the call to devm_kzalloc() in
probe().

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200914154601.32245-11-brgl@bgdev.pl