Some Toshiba models (most notably Qosmios) come with an
incomplete backlight method where the AML code doesn't
check for write or read commands and always returns
HCI_SUCCESS and the actual brightness (and in some
cases the max brightness), thus allowing the backlight
interface to be registered without write support.

This patch changes the set_lcd_brightness function,
checking the returned values for values greater than
zero to avoid registering a broken backlight interface.
Signed-off-by: Azael Avalos <coproscefalo@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>