leds: lm3692x: Return 0 from remove callback

The only difference between returning zero or a non-zero value is that
for the non-zero case the i2c will print a generic error message
("remove failed (-ESOMETHING), will be ignored").

In this case however the driver itself already emitted a more helpful
error message, so the additional error message isn't helpful at all.

The long-term goal is to make the i2c remove callback return void, making
all implementations return 0 is preparatory work for this change.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Pavel Machek <pavel@ucw.cz>