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

cfoch/gstreamer,surround-io/gstreamer,shelsonjava/gstreamer,lubosz/gstreamer,jpxiong/gstreamer,collects/gstreamer,collects/gstreamer,lovebug356/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,krichter722/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,mparis/gstreamer,jpakkane/gstreamer,collects/gstreamer,cablelabs/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,lubosz/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,mparis/gstreamer,collects/gstreamer,surround-io/gstreamer,centricular/gstreamer,lovebug356/gstreamer,surround-io/gstreamer,justinjoy/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,collects/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,krieger-od/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,justinjoy/gstreamer,centricular/gstreamer,cablelabs/gstreamer,krieger-od/gstreamer,krieger-od/gstreamer,cfoch/gstreamer,justinjoy/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,lubosz/gstreamer,lubosz/gstreamer,mparis/gstreamer,centricular/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,krichter722/gstreamer,centricular/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,krieger-od/gstreamer,lubosz/gstreamer,justinjoy/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,cfoch/gstreamer,surround-io/gstreamer,krichter722/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,justinjoy/gstreamer,lovebug356/gstreamer