[media] soc_camera: fix compiler warning

[media] soc_camera: fix compiler warning

media_build/v4l/soc_camera.c: In function 'soc_camera_host_register':
media_build/v4l/soc_camera.c:1513:10: warning: 'sasd' may be used uninitialized in this function [-Wmaybe-uninitialized]
  snprintf(clk_name, sizeof(clk_name), "%d-%04x",
          ^
media_build/v4l/soc_camera.c:1464:34: note: 'sasd' was declared here
  struct soc_camera_async_subdev *sasd;
                                  ^
By changing the type of 'i' to unsigned and changing a condition we finally
convince the compiler that sasd is really initialized.

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