greybus: lights: remove sync operation and work queue

In kernel v4.5 there is a change in LED api, which remove the need for
individual work queue and rename the set_sync operation to set_blocking.
This patch add the handling of this case and avoid compilation failure
for this kernel versions.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>