media: em28xx: make v4l2-compliance happier by starting sequence on zero

The v4l2-compliance tool complains if a video doesn't start
with a zero sequence number.

While this shouldn't cause any real problem for apps, let's
make it happier, in order to better check the v4l2-compliance
differences before and after patchsets.

This is actually an old issue. It is there since at least its
videobuf2 conversion, e. g. changeset 3829fadc461 ("[media]
em28xx: convert to videobuf2"), if VB1 wouldn't suffer from
the same issue.

Cc: stable@vger.kernel.org
Fixes: d3829fad ("[media] em28xx: convert to videobuf2")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>