media: staging/ipu7: Disallow source multiplexing

[vicamo]

v6.18-rc1 commit 5195b777552d2 ("media: v4l2-subdev: Make struct v4l2_subdev_stream_config private") stripped the use of v4l2_subdev_stream_config from the public. The "solution" should be same v6.18-rc1 commit 49cec2b5a316c (media: staging/ipu7: Disallow source multiplexing") that removed the use of stream_configs due to stream multiplexing.

drivers/media/pci/intel/ipu7/ipu7-isys-video.c: In function 'get_remote_pad_stream':
drivers/media/pci/intel/ipu7/ipu7-isys-video.c:884:55: error: invalid use of undefined type 'struct v4l2_subdev_stream_config'
  884 |                         &state->stream_configs.configs[i];
      |                                                       ^
drivers/media/pci/intel/ipu7/ipu7-isys-video.c:885:24: error: invalid use of undefined type 'struct v4l2_subdev_stream_config'
  885 |                 if (cfg->pad == r_pad->index) {
      |                        ^~
drivers/media/pci/intel/ipu7/ipu7-isys-video.c:886:40: error: invalid use of undefined type 'struct v4l2_subdev_stream_config'
  886 |                         stream_id = cfg->stream;
      |                                        ^~
make[5]: *** [/usr/src/linux-headers-6.18.0-9003-generic/scripts/Makefile.build:287: drivers/media/pci/intel/ipu7/ipu7-isys-video.o] Error 1