leds: lm3692x: Handle failure to probe the regulator

[ Upstream commit 396128d2 ]

Instead use devm_regulator_get_optional since the regulator
is optional and check for errors.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>