Since the of specific code are declared in <linux/of_mtd.h> regardless
of CONFIG_OF. Remove the #if defined(CONFIG_OF) guard and use an
IS_ENABLED(CONFIG_OF) instead.

Thanks to Ezequiel Garcia's for this protype.
Signed-off-by: NJosh Wu <josh.wu@atmel.com>
Acked-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>