Fix device capability checking in v4l2 input.

The capabilities flags that were used previously describe all
capabilities the physical device offers. This would cause devices
that are accessible through multiple device nodes to show up with
all device nodes while only one of the nodes might actually offer
the needed video capture capability.

If the device has more nodes the CAP_DEVICES_CAP flag might be set
in which case the device_caps field is filled with the capabilities
that only apply to that specific node that is opened.