deinterlace: Fix typo to not change the input caps but our filtered caps
deinterlace: Fix typo to not change the input caps but our filtered caps

Changing the input caps and not using them anymore afterwards is useless, and
it breaks negotiation in pipelines like:

gst-launch-1.0 videotestsrc ! "video/x-raw,framerate=25/1,interlace-mode=interleaved" !
  deinterlace fields=all ! "video/x-raw,framerate=50/1,interlace-mode=progressive" !
  fakesink

stfl/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good