typefindelement: if sink pad is activated do not change mode

typefindelement: if sink pad is activated do not change mode

In commit bf0964b6 a check for pad is activated was not carried.
This leads to attempt to pull while in push mode when force_caps
is set. In this case without the attached check even when activated
in pull mode we activate back to push mode.

This is from comment in previous code , case number eight:
 8. if the sink pad is activated, we are in pull mode. succeed.
-     otherwise activate both pads in push mode and succeed.

Putting it back fixes playback of webm in webkit+gstreamer 1.0 .

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=676003