[ARM] 5138/1: magician: set pwm-backlight .id = -1

There will always be only one pwm-backlight on this device.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>