msm: camera: validate num_streams in stream_cfg_cmd before using it

stream_cfg_cmd->num_streams is from userspace,
need to check it against MSM_ISP_STATS_MAX before using it.

CRs-Fixed: 2029867
Change-Id: I02a71b983947981806470454654d712bcc732077
Signed-off-by: Terence Ho <terenceh@codeaurora.org>