platform/x86: dell-laptop: Fix keyboard backlight timeout on XPS 13 9370

The XPS 13 9370 doesn't expose the necessary KBD_LED_AC_TOKEN in
the BIOS, so the driver thinks it cannot adjust the AC keyboard
backlight timeout. This patch adds a quirk to fix this until
Dell adds the missing token to the BIOS.

For further discussion, see:
https://github.com/dell/libsmbios/issues/48

Signed-off-by: Timur Kristóf <venemo@fedoraproject.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>