[media] v4l: Fix media_entity_to_video_device macro argument name

The name 'entity' is used twice in the macro body, once as the macro
argument, and once as a structure field name. This breaks compilation if
the macro is called with its argument not named 'entity'.

Fix this by renaming the macro argument '__e'. This should avoid
namespace clashes.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>