media: i2c: st-mipid02: use enable/disable_streams pad ops

Add pad enable_streams and disable_streams ops in addition to
v4l2_subdev_s_stream_helper. Moreover, use functions
v4l2_subdev_enable_streams and v4l2_subdev_disable_streams to
control the source subdev.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>