media: rcar_drif: use explicit name for bus_info

Fixes this compiler warning:

drivers/media/platform/renesas/rcar_drif.c: In function 'rcar_drif_querycap':
drivers/media/platform/renesas/rcar_drif.c:874:66: warning: '%s' directive output may be truncated writing up to 63 bytes into a region of size 23 [-Wformat-truncation=]
  874 |         snprintf(cap->bus_info, sizeof(cap->bus_info), "platform:%s",
      |                                                                  ^~
drivers/media/platform/renesas/rcar_drif.c:874:9: note: 'snprintf' output between 10 and 73 bytes into a destination of size 32
  874 |         snprintf(cap->bus_info, sizeof(cap->bus_info), "platform:%s",
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  875 |                  sdr->vdev->name);
      |                  ~~~~~~~~~~~~~~~~

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>