rtc: max77686: Use ARRAY_SIZE() instead of current array length

It is better to use the ARRAY_SIZE() macro instead of the array length
to avoid bugs if the array is later changed and the length not updated.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>