toshiba_acpi: Only register backlight device when interface is read/write

Currently the backlight device is registered unconditionally, but many
(probably most) Toshibas either don't support HCI_LCD_BRIGHTNESS or only
support reading from it. This patch adds a test of HCI_LCD_BRIGHTNESS
during initialization and only registers the backlight device if this
interface supports both reads and writes.

Cc: Akio Idehara <zbe64533@gmail.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>