leds: led-class.c - Quiet boot messages

As each led device gets registered a kernel message gets printed. In
an embedded system with a number of leds this can produce a lot
of output that just looks like noise.

Change the message type to KERN_DEBUG since it might be useful
in the dmesg output "after" booting.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>