platform/x86: dell-laptop: Use brightness_set_blocking for kbd_led_level_set

kbd_led_level_set uses dell_smbios call which blocks, so the kbd_led
classdev should use the brightness_set_blocking callback.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>