lib/string_choices: Add str_write_read() helper

Add an inversed variant of str_read_write(), i.e. str_write_read().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230703185222.50554-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>