v4l2src: use _negotiate rather than _fixate
v4l2src: use _negotiate rather than _fixate

The latter might fixate parameters to values that do not describe the stream.
In fact, there is not much fixating or negotiating since _setup already
determined all stream parameters, so we need caps to describe those.

Josuercuevas/gstreamer-imx,commshare/gstreamer-imx,Josuercuevas/gstreamer-imx,commshare/gstreamer-imx