media: bcm2835-unicam: Forward input status from subdevice

media: bcm2835-unicam: Forward input status from subdevice

The vidioc_enum_input() v4l2 ioctl is capable of returning
sensor/input status as well. This is used in current
GStreamer HEAD for signal detection [1].

bcm2835-unicam does handle this syscall, but it didn't ask
the subdevice driver about the input status. The input then
appeared as always present.

This commit adds the necessary query. There is a precedent for
this - the R-Car VIN V4L2 driver does a similar call [2].

[1]: https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/blob/ce0be27caf69aa9d96b73bc2b50737451b6f6936/sys/v4l2/gstv4l2src.c#L553
[2]: https://github.com/raspberrypi/linux/blob/7fb9d006d3ff3baf2e205e0c85c4e4fd0a64fcd0/drivers/media/platform/rcar-vin/rcar-v4l2.c#L548

Signed-off-by: Jakub Vaněk <linuxtardis@gmail.com>