When not all LED channels of the led chip are configured, the
sysfs selftest functionality gives erroneous results and tries to
test all channels of the chip.
There is a potential for LED overcurrent conditions since the
test current will be set to values from out-of-bound regions.

It is wrong to use pdata->led_config[i].led_current to skip absent
channels as led_config[] only contains the configured LED channels.

Instead of iterating over all the physical channels of the device,
loop over the available LED configurations and use led->chan_nr to
access the correct i2c registers. Keep the zero-check for the LED
current as existing users might depend on this to disable a channel.
Reported-by: Arne Staessen <a.staessen@televic.com>
Signed-off-by: Maarten Zanders <maarten.zanders@mind.be>
Signed-off-by: Pavel Machek <pavel@ucw.cz>