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.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>