[media] mem2mem_testdev: fix field, sequence and time copying

- Set the sequence counters correctly.
- Copy timestamps, timecode, relevant buffer flags and field from
  the received buffer to the outgoing buffer.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>