media: microchip: microchip-isc: move media_pipeline_* to (un)prepare cb

Move the media_pipeline_start/stop calls from start/stop streaming to
the new prepare_streaming and unprepare_streaming callbacks.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>