As of commit c7fe3bbfd622b5ee ('mfd: core: Use of_property_read_reg() to
parse "reg"'), all code in mfd_match_of_node_to_dev() compiles fine when
CONFIG_OF is disabled. As the sole caller of this function is
protected by IS_ENABLED(CONFIG_OF), the #ifdef inside the function can
be removed to increase build coverage, without impacting code size.