V4L/DVB (12406): em28xx: fix: don't do image interlacing on webcams

Due to historical reasons, em28xx driver gets two consecutive frames and
fold them into an unique framing, doing interlacing. While this works
fine for TV images, this produces two bad effects with webcams:

1) webcam images are progressive. Merging two consecutive images produce
interlacing artifacts on the image;

2) since the driver needs to get two frames, it reduces the maximum
frame rate by two.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>