media: staging/imx: fill vb2_v4l2_buffer field entry

- fixes gstreamer v4l2src warning:

  0:00:00.716640334  349  0x164f720 WARN  v4l2bufferpool gstv4l2bufferpool.c:1195:gst_v4l2_buffer_pool_dqbuf:<v4l2src0:pool:src> Driver should never set v4l2_buffer.field to ANY

- fixes v4l2-compliance test failure:

  Streaming ioctls:
          test read/write: OK (Not Supported)
              Video Capture:
                  Buffer: 0 Sequence: 0 Field: Any Timestamp: 58.383658s
                  fail: v4l2-test-buffers.cpp(297): g_field() == V4L2_FIELD_ANY

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>