media: stm32-dcmi: improve sensor subdev naming

Rename "subdev" entity struct field to "source"
to prepare for several subdev support.
Move asd field on top of entity struct.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>