platform/x86: thinkpad_acpi: Revert "Use strndup_user() in dispatch_proc_write()"

This reverts commit 35d13c7a.

This broke procfs interface due to neglecting the fact that
the strings are not coming NULL terminated.

Revert the change till we will have a better clean up.

Fixes: 35d13c7a

 ("platform/x86: thinkpad_acpi: Use strndup_user() in dispatch_proc_write()")
Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>