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.
v4l2src: also handle tv capture source

... by adding some calls that seem needed to convince some driver(s)
to get it right.
Merge pull request #12 from mnauw/v4l2src-tweaks

v4l2src tweaks