v4l2_get_device_list always returns zero

[unusedsong]

In doc for avdevice_list_input_sources we can see:
@return count of autodetected devices, negative on error.

But get_device_list for v4l2 returns zero on success.
It's a little confusing.
Perhaps there should be a pointing to nb_devices field.

[Marton Balint]

Fixed in 64f04df37942c1b1fc11df0f99ffba7834c33f34.