mfd: max8997: Use "IS_ENABLED(CONFIG_OF)" for DT code.

Instead of "#if define CONFIG_OF" use "IS_ENABLED(CONFIG_OF)"
option for DT code to avoid if-deffery in code.

Signed-off-by: Manish Badarkhe <badarkhe.manish@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>