v4l2: cast unused return to void
v4l2: cast unused return to void

Quell unchecked return value defect by casting the return value to void and
making it explicit it is going to be ignored.

CID #206031

sebras/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,chamois94/gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,stfl/gst-plugins-good,froggatt/gst-plugins-good-m,chamois94/gst-plugins-good,vatavuserban/gst-plugins-good,ikonst/gst-plugins-good,shelsonjava/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,chamois94/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,sebras/gst-plugins-good,GrokImageCompression/gst-plugins-good,vatavuserban/gst-plugins-good,rawoul/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,kittee/gst-plugins-good,vatavuserban/gst-plugins-good,kittee/gst-plugins-good,stfl/gst-plugins-good,vatavuserban/gst-plugins-good,kittee/gst-plugins-good,froggatt/gst-plugins-good-m,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,shelsonjava/gst-plugins-good,kittee/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,shelsonjava/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,sebras/gst-plugins-good,rawoul/gst-plugins-good,chamois94/gst-plugins-good,stfl/gst-plugins-good,shelsonjava/gst-plugins-good,froggatt/gst-plugins-good-m,froggatt/gst-plugins-good-m,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,ikonst/gst-plugins-good,GrokImageCompression/gst-plugins-good