iio: dac: ad5592r: Drop leftover header inclusion

There is no use of of.h in the module, drop it.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220203123354.76573-1-andriy.shevchenko@linux.intel.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5592r-base.c

@@ -11,7 +11,6 @@
 #include <linux/iio/iio.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
-#include <linux/of.h>
 #include <linux/regulator/consumer.h>
 #include <linux/gpio/consumer.h>
 #include <linux/gpio/driver.h>