V4L/DVB (6272): V4L: properly fix support for capturing interlaced video in saa7134

By "capturing interlaced video" I mean that card ensures that top field
is really top and vice versa (I think it takes the filed ID from signal)
Properly turn on/off that support depending on signal state

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>