According to API docs avdevice_list_devices(), avdevice_list_input_sources()
and avdevice_list_input_sinks() should return the number of autodetected
devices on success. This is redundant with AVDeviceInfoList->nb_devices so it
was not noticed earlier that none of the underlying device list functions work
like that.

Let's fix it in generic code to make it in line with the API docs.

Fixes ticket #9820.
Signed-off-by: Marton Balint <cus@passwd.hu>