[media] cx231xx: make video scaler work properly

Move the responsibility for setting up the horizontal and vertical scalers
entirely to the cx25840 driver.  The cx231xx-avcore was actually programming
garbage into the HSCALE_CTRL and VSCALE_CTRL registers (because of differences
in how the em28xx driver worked, which the cx231xx driver was derived from).

The net effect is that the scaler now works properly (tested with both PAL
and NTSC under mplayer and tvtime).

This patch also gets rid of cx25840 errors showing up in dmesg which say
"720x480 is not a valid size" (since we now properly setup the size of the
active video area).

Signed-off-by: Devin Heitmueller <dheitmueller@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>