media: v4l: async: Don't set sd->dev NULL in v4l2_async_cleanup

v4l2_async_cleanup() is called when the async sub-device is unbound from
the media device. As the pointer is set by the driver registering the
async sub-device, leave the pointer as set by the driver.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>