media: staging/imx: fill vb2_v4l2_buffer sequence entry

- enables gstreamer v4l2src lost frame detection, e.g:

  0:00:08.685185668  348  0x54f520 WARN  v4l2src gstv4l2src.c:970:gst_v4l2src_create:<v4l2src0> lost frames detected: count = 141 - ts: 0:00:08.330177332

- fixes v4l2-compliance test failure:

  Streaming ioctls:
          test read/write: OK (Not Supported)
              Video Capture:
                  Buffer: 0 Sequence: 0 Field: None Timestamp: 92.991450s
                  Buffer: 1 Sequence: 0 Field: None Timestamp: 93.008135s
                  fail: v4l2-test-buffers.cpp(294): (int)g_sequence() < seq.last_seq + 1
                  fail: v4l2-test-buffers.cpp(707): buf.check(q, last_seq)

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>