msm: camera: do not return when duration property is unavailable

qcom,duration property may not be specified on led flash nodes.
Treat it as an optional property. Do not return error when it is not
present on flash nodes.

Change-Id: Ie93de7862f78dc20c04de2f3b7a9aa45b8dc1424
Signed-off-by: Xu Han <hanxu@codeaurora.org>