leds: lm3692x: Print error value on dev_err

This gives a way better idea what is going on.

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