backlight: lp8788_bl: Delete a check before backlight_device_unregister()

The backlight_device_unregister() function tests whether its argument
is NULL and then returns immediately. Thus the test around the call is
not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>