platform/chrome: cros_kbd_led_backlight: separate ACPI backend

cros_kbd_led_backlight uses ACPI_KEYBOARD_BACKLIGHT_WRITE and
ACPI_KEYBOARD_BACKLIGHT_READ for setting and getting the brightness
respectively.

Separate ACPI operations for preparing the driver to support other
backends.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220523090822.3035189-3-tzungbi@kernel.org