kstrtox() may return different error codes.

Unshadow them in the ->store() callback to give better error report.

While at it, add missing kstrtox.h inclusion.
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>