[media] V4L2: soc-camera: fix uninitialised use compiler warning

[media] V4L2: soc-camera: fix uninitialised use compiler warning

In scan_async_group() if the size parameter is negative, the sasd pointer
will be used uninitialised:
drivers/media/platform/soc_camera/soc_camera.c: In function "soc_camera_host_register":
drivers/media/platform/soc_camera/soc_camera.c:1514:55: warning: "sasd" may
be used uninitialized in this function [-Wmaybe-uninitialized]
    sasd->asd.match.i2c.adapter_id, sasd->asd.match.i2c.address);
                                                       ^
drivers/media/platform/soc_camera/soc_camera.c:1464:34: note: "sasd" was
declared here
  struct soc_camera_async_subdev *sasd;
Fix this by making "size" and the array, from which it is assigned unsigned.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>