V4L/DVB (12239): em28xx: fix webcam scaling

While trying to fix an mt9v001 webcam, I noticed that HSCALE/VSCALE do
work with em28xx + webcam. The issue is that the scaling setup depends
on the number of visible rows/cols of the input image.

With mt9v011 (Silvercrest), the resolution is 640x480. So, the scaling
is different from a normal TV image (720x480 on NTSC). This were causing
a wrong scaling and a previous patch disabled scaling.

As each sensor have their different resolution setting, the xres/yres
should be adjusted accordingly with the input sensor.

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