iio: dac: stm32-dac: Use correct header(s) instead of string_helpers.h

There is nothing from string_helpers.h used in the driver, correct
the header inclusion block accordingly.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230808164137.66663-1-andriy.shevchenko@linux.intel.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/stm32-dac.c

@@ -11,12 +11,13 @@
 #include <linux/delay.h>
 #include <linux/iio/iio.h>
 #include <linux/kernel.h>
+#include <linux/kstrtox.h>
 #include <linux/module.h>
 #include <linux/mod_devicetable.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
-#include <linux/string_helpers.h>
+#include <linux/string_choices.h>
 
 #include "stm32-dac-core.h"