video: backlight: tosa_lcd: drop check because i2c_unregister_device() is NULL safe

No need to check the argument of i2c_unregister_device() because the
function itself does it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>