kstrtox: consistently use _tolower()

We already use _tolower() in other places, so convert the one which open
codes it.

Link: https://lkml.kernel.org/r/20230817145919.543251-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>