media: platform: Remove unnecessary (void*) conversions

No need cast (void*) to (struct stdemux *),(struct hva_h264_ctx *)
or (struct hva_h264_task *).

Signed-off-by: Su Hui <suhui@nfschina.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>