backlight: lm3509_bl: Fix NULL vs IS_ERR() check in register() function

The devm_backlight_device_register() doesn't return NULL, it returns
error pointers.  Update the error checking to match.

Fixes: b72755f5 ("backlight: Add new lm3509 backlight driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/187b5bc5-a010-46c2-8ead-980df9efae79@moroto.mountain


Signed-off-by: Lee Jones <lee@kernel.org>