rtc: s5m: check the return value of s5m8767_rtc_init_reg()

This function can fail if regmap operations fail so check its return
value in probe().

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210114102219.23682-4-brgl@bgdev.pl