leds: lm3692x: Move lm3692x_init and rename to lm3692x_leds_enable

This moves lm3692x_init so it can be used from
lm3692x_brightness_set. Rename to lm3692_leds_enable to pair up
with lm3692x_leds_disable. No functional change.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>