v4l2src: fix first input used is always used next times

v4l2src: fix first input used is always used next times

The input from an v4l2 device that was used the first time was
remembered for next times, and set again always the pipeline is
set to READY state. This was making that users wasn't able to
select a different input without having to create a new pipeline.

This patch makes that v4l2src element forget previous used input
when going to NULL state, so it will check again for the current
selected input when going again to READY state. Users can change
to NULL state, select a new input with a VIDIOC_S_INPUT ioctl
and change to PLAYING again.

https://bugzilla.gnome.org/show_bug.cgi?id=796908