decklinkvideosrc: add missing break in set_property-code

When gst_decklink_video_src_set_property of the decklinkvideosrc-Element is called with a property_id of PROP_VIDEO_FORMAT, the switch/case-Block will fall through to the next case, because of a missing/misplaced break.

Before !214 (merged) it would fall through to PROP_TIMECODE_FORMAT which would've been set to 0. This would also be wrong but not produce any followup error.

With the addition of !214 (merged) the switch/case now falls through to PROP_DUPLEX_MODE instead, which is also set to 0. Contrary to timecode_format the Value of the duplex_mode-Property is checked for being either bmdDuplexModeFull or bmdDuplexModeHalf. The Value of 0 is neither of them, so this check now raises the crash reported in #936 (closed).