[media] usbtv: Fix deinterlacing

The image data is laid out a bit more weirdly and thus needs more work to
properly interlace. What we get from hardware is V4L2_FIELD_ALTERNATE, but
since userspace support for it is practically nonexistent, thus we make
V4L2_FIELD_INTERLACED from it so that it's more easily interpreted.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>