leds: lm3642: remove warnings for bad strtol, cleanup gotos

Bad string from userspace is not worth printing warning on; gotos that
just directly return are not good, plus a comment fix.

Cc: jacek.anaszewski@gmail.com, linux-leds@vger.kernel.org, linux-kernel@vger.kernel.org, dmurphy@ti.com
Signed-off-by: Pavel Machek <pavel@ucw.cz>