Please silence media/platform/mxc related warnings

I thought i broke something when building imx8-current-librem5 but then it was the camera:

drivers/media/platform/mxc/capture/mx6s_capture.c: In function ‘mx6s_vidioc_g_parm’:
drivers/media/platform/mxc/capture/mx6s_capture.c:1642:21: warning: unused variable ‘timeperframe’ [-Wunused-variable]
 1642 |  struct v4l2_fract *timeperframe = &a->parm.capture.timeperframe;
      |                     ^~~~~~~~~~~~
 ...
drivers/media/platform/mxc/capture/ov5640_mipi_v2.c: In function ‘OV5640_get_sysclk’:
drivers/media/platform/mxc/capture/ov5640_mipi_v2.c:731:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
  731 |  int sclk_rdiv_map[] = {1, 2, 4, 8};
      |  ^~~

Would be coo to have fixed since this makes one miss errors in local builds. @martin.kepplinger since you're on that driver, could you fix that please? I take all the blame since i should have checked with -Werror before merging.